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

Fixes #135

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

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

@@ -3,6 +3,7 @@
 namespace Box\Spout\Writer\ODS\Helper;
 
 use Box\Spout\Writer\Common\Helper\AbstractStyleHelper;
+use Box\Spout\Writer\Style\BorderPart;
 
 /**
  * Class StyleHelper
@@ -256,9 +257,16 @@ EOD;
             $content .= '<style:table-cell-properties fo:wrap-option="wrap" style:vertical-align="automatic"/>';
         }
 
+        if ($style->shouldApplyBorder()) {
+            $borderProperty = '<style:table-cell-properties %s />';
+            $borders = array_map(function (BorderPart $borderPart) {
+                return BorderHelper::serializeBorderPart($borderPart);
+            }, $style->getBorder()->getParts());
+            $content .= sprintf($borderProperty, implode(' ', $borders));
+        }
+
         $content .= '</style:style>';
 
         return $content;
     }
-
 }