Fix marijnh#153 Cover image doesn't show up with epub in iBooks (marijnh#237)

Add meta property cover in epub content.opf

Author: clemlatz

epub/content.opf.src

@@ -12,6 +12,7 @@
     <link rel="cc:license" href="http://creativecommons.org/licenses/by-nc/3.0/"/>
     <meta property="cc:attributionURL">http://eloquentjavascript.net/</meta>
     <meta property="dcterms:modified">2014-09-04T10:47:00Z</meta>
+    <meta name="cover" content="img/cover.png" />
   </metadata>
   <manifest>
     <item id="titlepage" href="titlepage.xhtml" media-type="application/xhtml+xml"/>