Disable HTTP caching for Admin API

This is a potential fix for #175.

Previously, we were sending a `Cache-Control: no-cache` header. However, to be more thorough and aggressive in cache busting, the following is generally considered necessary:

```HTTP
Expires: 0
Last-Modified: Tue, 12 Aug 2014 13:30:09 GMT
Cache-Control: no-store, no-cache, must-revalidate
Cache-Control: post-check=0, pre-check=0
Pragma: no-cache
```

This patch fires during the finish event, if the request was a GET or a HEAD request, sending the above headers (with the  Last-Modified header being set to the current timestamp).