Option to return formatted dates instead of PHP objects (#226)

When reading spreadsheets, Spout should be able to return formatted dates, as shown when opened with Excel for instance.
It currently only returns DateTime/DateInterval objects, making it impossible to read + write, as the Writer does not accept objects.

tests/Spout/Reader/XLSX/Helper/DateFormatHelperTest.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace Box\Spout\Reader\XLSX\Helper;
+
+/**
+ * Class DateFormatHelperTest
+ *
+ * @package Box\Spout\Reader\XLSX\Helper
+ */
+class DateFormatHelperTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * @return array
+     */
+    public function dataProviderForTestToPHPDateFormat()
+    {
+        return [
+            // Excel date format, expected PHP date format
+            ['m/d/yy hh:mm', 'n/j/y H:i'],
+            ['mmm-yy', 'M-y'],
+            ['d-mmm-yy', 'j-M-y'],
+            ['m/dd/yyyy', 'n/d/Y'],
+            ['e mmmmm dddd', 'Y M l'],
+            ['MMMM DDD', 'F D'],
+            ['hh:mm:ss.s', 'H:i:s'],
+            ['h:mm:ss AM/PM', 'g:i:s A'],
+            ['hh:mm AM/PM', 'h:i A'],
+            ['[$-409]hh:mm AM/PM', 'h:i A'],
+            ['[$USD-F480]hh:mm AM/PM', 'h:i A'],
+            ['"Day " d', '\\D\\a\\y\\  j'],
+            ['yy "Year" m "Month"', 'y \\Y\\e\\a\\r n \\M\\o\\n\\t\\h'],
+        ];
+    }
+
+    /**
+     * @dataProvider dataProviderForTestToPHPDateFormat
+     *
+     * @param string $excelDateFormat
+     * @param string $expectedPHPDateFormat
+     * @return void
+     */
+    public function testToPHPDateFormat($excelDateFormat, $expectedPHPDateFormat)
+    {
+        $phpDateFormat = DateFormatHelper::toPHPDateFormat($excelDateFormat);
+        $this->assertEquals($expectedPHPDateFormat, $phpDateFormat);
+    }
+}