Allow requesting badges for branches != master

This changed the cache structure to allow caching data for multiple
branches. The service still assumes a default branch named `master` but
using the `branch` get parameter, the default branch can be changed.

The 404 page for the missing `master` branch was changed to explain the
new parameter.

templates/index.rs.html

@@ -45,6 +45,12 @@ would render this badge:
 alt="example badge" /></a>
 </pre>
 
+<p>
+By default, this service assumes the existence of a branch named <code>master</code>. If no branch with that name exists
+in your repository or you want a badge for another branch of your repository, just append
+<code>?branch=&lt;branch-name&gt;</code> to the URL.
+</p>
+
 <p>
 You can also request the HoC as JSON by appending <code>/json</code> to the request path. This will return a JSON object
 with three fields: <code>count</code> (the HoC value), <code>commits</code> (the number of commits) and