Badge caching

Related to #145.

This institutes a sort-of reverse proxy for badges to try and get around the problems with Github's image proxy.

The `Expires` header is set so the Github proxy does not cache the image.
`Last-Modified` is set to the commit's `Update` time and `If-Modified-Since` will be obeyed (image won't be re-requested from shields.io).
@caffinatedmonkey See [this comment](https://github.com/github/markup/issues/224#issuecomment-37663375). It seems as if Fastly doesn't properly revalidate with just `no-cache`.

[Even though Last-Modified should be enough in this instance](http://tools.ietf.org/html/rfc7234#section-4.3.1) in my tests it doesn't seem to be the case.

If anybody knows how to deal with this properly I'm all ears.