Support for strict OOXML

There are 2 types of OOXML format: transitional and strict. Transitional is what's mostly used but some softwares still allow XLSX to be generated using the strict OOXML format.
In this format, namespaces of the XML files are different: `http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings` is replaced by `http://purl.oclc.org/ooxml/officeDocument/relationships/sharedStrings` for instance. To support both formats, Spout needs to be able to look for both.

tests/Spout/Reader/XLSX/ReaderTest.php

@@ -703,6 +703,18 @@ class ReaderTest extends TestCase
         $this->assertEquals($expectedRows, $allRows, 'Cell values should not be trimmed');
     }
 
+    /**
+     * https://github.com/box/spout/issues/726
+     * @return void
+     */
+    public function testReaderShouldSupportStrictOOXML()
+    {
+        $allRows = $this->getAllRowsForFile('sheet_with_strict_ooxml.xlsx');
+
+        $this->assertEquals('UNIQUE_ACCOUNT_IDENTIFIER', $allRows[0][0]);
+        $this->assertEquals('A2Z34NJA7N2ESJ', $allRows[1][0]);
+    }
+
     /**
      * @param string $fileName
      * @param bool $shouldFormatDates