First stab at #135 - Add borders (#200)

Fixes #135

Added border support for ODS and XLSX files.
Updated README.

src/Spout/Writer/XLSX/Helper/BorderHelper.php

@@ -0,0 +1,68 @@
+<?php
+
+namespace Box\Spout\Writer\XLSX\Helper;
+
+use Box\Spout\Writer\Style\Border;
+use Box\Spout\Writer\Style\BorderPart;
+
+class BorderHelper
+{
+    public static $xlsxStyleMap = [
+        Border::STYLE_SOLID => [
+            Border::WIDTH_THIN => 'thin',
+            Border::WIDTH_MEDIUM => 'medium',
+            Border::WIDTH_THICK => 'thick'
+        ],
+        Border::STYLE_DOTTED => [
+            Border::WIDTH_THIN => 'dotted',
+            Border::WIDTH_MEDIUM => 'dotted',
+            Border::WIDTH_THICK => 'dotted',
+        ],
+        Border::STYLE_DASHED => [
+            Border::WIDTH_THIN => 'dashed',
+            Border::WIDTH_MEDIUM => 'mediumDashed',
+            Border::WIDTH_THICK => 'mediumDashed',
+        ],
+        Border::STYLE_DOUBLE => [
+            Border::WIDTH_THIN => 'double',
+            Border::WIDTH_MEDIUM => 'double',
+            Border::WIDTH_THICK => 'double',
+        ],
+        Border::STYLE_NONE => [
+            Border::WIDTH_THIN => 'none',
+            Border::WIDTH_MEDIUM => 'none',
+            Border::WIDTH_THICK => 'none',
+        ],
+    ];
+
+    /**
+     * @param BorderPart $borderPart
+     * @return string
+     */
+    public static function serializeBorderPart(BorderPart $borderPart)
+    {
+        $borderStyle = self::getBorderStyle($borderPart);
+
+        $colorEl = $borderPart->getColor() ? sprintf('<color rgb="%s"/>', $borderPart->getColor()) : '';
+        $partEl = sprintf(
+            '<%s style="%s">%s</%s>',
+            $borderPart->getName(),
+            $borderStyle,
+            $colorEl,
+            $borderPart->getName()
+        );
+
+        return $partEl . PHP_EOL;
+    }
+
+    /**
+     * Get the style definition from the style map
+     *
+     * @param BorderPart $borderPart
+     * @return string
+     */
+    protected static function getBorderStyle(BorderPart $borderPart)
+    {
+        return self::$xlsxStyleMap[$borderPart->getStyle()][$borderPart->getWidth()];
+    }
+}