Add support for styling

Added top level methods on the Writer:
 - addRowWithStyle()
 - addRowsWithStyle()

Added a style builder, to easily create new styles.
Each writer can specify its own default style and all styles will automatically inherit from it.

For now, the style properties supported are:
 - bold
 - italic
 - underline
 - strikethrough
 - font size
 - font name
 - wrap text (alignment)

tests/Spout/Writer/XLSX/Helper/StyleHelperTest.php

@@ -0,0 +1,59 @@
+<?php
+
+namespace Box\Spout\Writer\XLSX\Helper;
+
+use Box\Spout\Writer\Style\StyleBuilder;
+
+/**
+ * Class StyleHelperTest
+ *
+ * @package Box\Spout\Writer\XLSX\Helper
+ */
+class StyleHelperTest extends \PHPUnit_Framework_TestCase
+{
+    /** @var \Box\Spout\Writer\Style\Style */
+    protected $defaultStyle;
+
+    /**
+     * @return void
+     */
+    public function setUp()
+    {
+        $this->defaultStyle = (new StyleBuilder())->build();
+    }
+
+    /**
+     * @return void
+     */
+    public function testRegisterStyleShouldUpdateId()
+    {
+        $style1 = (new StyleBuilder())->setFontBold()->build();
+        $style2 = (new StyleBuilder())->setFontUnderline()->build();
+
+        $this->assertEquals(0, $this->defaultStyle->getId(), 'Default style ID should be 0');
+        $this->assertNull($style1->getId());
+        $this->assertNull($style2->getId());
+
+        $styleHelper = new StyleHelper($this->defaultStyle);
+        $registeredStyle1 = $styleHelper->registerStyle($style1);
+        $registeredStyle2 = $styleHelper->registerStyle($style2);
+
+        $this->assertEquals(1, $registeredStyle1->getId());
+        $this->assertEquals(2, $registeredStyle2->getId());
+    }
+
+    /**
+     * @return void
+     */
+    public function testRegisterStyleShouldReuseAlreadyRegisteredStyles()
+    {
+        $style = (new StyleBuilder())->setFontBold()->build();
+
+        $styleHelper = new StyleHelper($this->defaultStyle);
+        $registeredStyle1 = $styleHelper->registerStyle($style);
+        $registeredStyle2 = $styleHelper->registerStyle($style);
+
+        $this->assertEquals(1, $registeredStyle1->getId());
+        $this->assertEquals(1, $registeredStyle2->getId());
+    }
+}