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

Fixes #135

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

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

@@ -0,0 +1,68 @@
+<?php
+
+namespace Box\Spout\Writer\ODS\Helper;
+
+use Box\Spout\Writer\Style\BorderPart;
+use Box\Spout\Writer\Style\Border;
+
+/**
+ * Class BorderHelper
+ *
+ * The fo:border, fo:border-top, fo:border-bottom, fo:border-left and fo:border-right attributes
+ * specify border properties
+ * http://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part1.html#__RefHeading__1419780_253892949
+ *
+ * Example table-cell-properties
+ *
+ * <style:table-cell-properties
+ * fo:border-bottom="0.74pt solid #ffc000" style:diagonal-bl-tr="none"
+ * style:diagonal-tl-br="none" fo:border-left="none" fo:border-right="none"
+ * style:rotation-align="none" fo:border-top="none"/>
+ */
+class BorderHelper
+{
+    /**
+     * Width mappings
+     *
+     * @var array
+     */
+    protected static $widthMap = [
+        Border::WIDTH_THIN => '0.75pt',
+        Border::WIDTH_MEDIUM => '1.75pt',
+        Border::WIDTH_THICK => '2.5pt',
+    ];
+
+    /**
+     * Style mapping
+     *
+     * @var array
+     */
+    protected static $styleMap = [
+        Border::STYLE_SOLID => 'solid',
+        Border::STYLE_DASHED => 'dashed',
+        Border::STYLE_DOTTED => 'dotted',
+        Border::STYLE_DOUBLE => 'double',
+    ];
+
+    /**
+     * @param BorderPart $borderPart
+     * @return string
+     */
+    public static function serializeBorderPart(BorderPart $borderPart)
+    {
+        $definition = 'fo:border-%s="%s"';
+
+        if ($borderPart->getStyle() === Border::STYLE_NONE) {
+            $borderPartDefinition = sprintf($definition, $borderPart->getName(), 'none');
+        } else {
+            $attributes = [
+                self::$widthMap[$borderPart->getWidth()],
+                self::$styleMap[$borderPart->getStyle()],
+                '#' . $borderPart->getColor(),
+            ];
+            $borderPartDefinition = sprintf($definition, $borderPart->getName(), implode(' ', $attributes));
+        }
+
+        return $borderPartDefinition;
+    }
+}