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

Fixes #135

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

src/Spout/Writer/Exception/Border/InvalidStyleException.php

@@ -0,0 +1,16 @@
+<?php
+
+namespace Box\Spout\Writer\Exception\Border;
+
+use Box\Spout\Writer\Exception\WriterException;
+use Box\Spout\Writer\Style\BorderPart;
+
+class InvalidStyleException extends WriterException
+{
+    public function __construct($name)
+    {
+        $msg = '%s is not a valid style identifier for a border. Valid identifiers are: %s.';
+
+        parent::__construct(sprintf($msg, $name, implode(',', BorderPart::getAllowedStyles())));
+    }
+}