Fix shared strings XML Entities auto decode (#411)

When converting an XMLReader node to a SimpleXMLElement, the conversion would automatically decode the XML entities. This resulted in a double decode.
For example: """ was converted to """ when imported into a SimpleXMLElement and was again converted into " (quote).

This commit changes the way the XLSX Shared Strings file is processed. It also changes the unescaping logic for both XLSX and ODS.

Finally, it removes any usage of the SimpleXML library (yay!).

tests/Spout/Reader/Wrapper/SimpleXMLElementTest.php

@@ -1,127 +0,0 @@
-<?php
-
-namespace Box\Spout\Reader\Wrapper;
-
-use Box\Spout\TestUsingResource;
-
-/**
- * Class SimpleXMLElementTest
- *
- * @package Box\Spout\Reader\Wrapper
- */
-class SimpleXMLElementTest extends \PHPUnit_Framework_TestCase
-{
-    use TestUsingResource;
-
-    /**
-     * @expectedException \Box\Spout\Reader\Exception\XMLProcessingException
-     *
-     * @return void
-     */
-    public function testConstructShouldThrowExceptionIfInvalidData()
-    {
-        $invalidXML = '<invalid><xml></invalid>';
-        new SimpleXMLElement($invalidXML);
-    }
-
-    /**
-     * @return array
-     */
-    public function dataProviderForTestGetAttribute()
-    {
-        $xmlWithoutNamespace = <<<XML
-<?xml version="1.0" encoding="UTF-8"?>
-<worksheet foo="bar" type="test" />
-XML;
-
-        $xmlWithHalfNamespace = <<<XML
-<?xml version="1.0" encoding="UTF-8"?>
-<worksheet
-    xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"
-    foo="bar" r:type="test" />
-XML;
-
-        $xmlWithFullNamespace = <<<XML
-<?xml version="1.0" encoding="UTF-8"?>
-<worksheet
-    xmlns="http://schemas.openxmlformats.org/spreadsheetml/2006/main" xmlns:r="http://schemas.openxmlformats.org/officeDocument/2006/relationships"
-    r:foo="bar" r:type="test" />
-XML;
-
-        return [
-            [$xmlWithoutNamespace, null, ['foo' => 'bar', 'type' => 'test']],
-            [$xmlWithHalfNamespace, null, ['foo' => 'bar', 'type' => null]],
-            [$xmlWithFullNamespace, null, ['foo' => null, 'type' => null]],
-            [$xmlWithoutNamespace, 'r', ['foo' => null, 'type' => null]],
-            [$xmlWithHalfNamespace, 'r', ['foo' => null, 'type' => 'test']],
-            [$xmlWithFullNamespace, 'r', ['foo' => 'bar', 'type' => 'test']],
-        ];
-    }
-
-    /**
-     * @dataProvider dataProviderForTestGetAttribute
-     *
-     * @param string $xml
-     * @param string|null $namespace
-     * @param array $expectedAttributes
-     * @return void
-     */
-    public function testGetAttribute($xml, $namespace, $expectedAttributes)
-    {
-        $element = new SimpleXMLElement($xml);
-
-        foreach ($expectedAttributes as $name => $expectedValue) {
-            $value = $element->getAttribute($name, $namespace);
-            $this->assertEquals($expectedValue, $value);
-        }
-    }
-
-    /**
-     * @return void
-     */
-    public function testXPath()
-    {
-        $xml = <<<XML
-<?xml version="1.0" encoding="UTF-8"?>
-<worksheet>
-    <sheetData>
-        <row r="1">
-            <c r="A1"><v>0</v></c>
-            <c r="A2"><v>1</v></c>
-        </row>
-    </sheetData>
-</worksheet>
-XML;
-
-        $element = new SimpleXMLElement($xml);
-        $matchedElements = $element->xpath('//c');
-
-        $this->assertEquals(2, count($matchedElements));
-        $this->assertTrue($matchedElements[0] instanceof SimpleXMLElement, 'The SimpleXMLElement should be wrapped');
-        $this->assertEquals('A2', $matchedElements[1]->getAttribute('r'));
-    }
-
-    /**
-     * @return void
-     */
-    public function testRemoveNodeMatchingXPath()
-    {
-        $xml = <<<XML
-<?xml version="1.0" encoding="UTF-8"?>
-<worksheet>
-    <sheetData>
-        <row r="1">
-            <c r="A1"><v>0</v></c>
-            <c r="A2"><v>1</v></c>
-        </row>
-    </sheetData>
-</worksheet>
-XML;
-
-        $element = new SimpleXMLElement($xml);
-        $this->assertNotNull($element->getFirstChildByTagName('sheetData'));
-
-        $element->removeNodesMatchingXPath('//sheetData');
-        $this->assertNull($element->getFirstChildByTagName('sheetData'));
-    }
-}