Delete entries from the cache after PUT/POST/DELETE requests.

A first stab on #42.

Some things that need to polishing:

* [ ] Log messages aren't helpful. Right now every request will be logged with `unacceptable, delete`. I'm not sure if we should check if the request was cached before to ensure that only real deletes gets logged as `delete`.
* [ ] As we cache both `GET` and `HEAD` differently, we need to try to delete both kind of requests in the same way. I'm not sure if this is the time to normalize that and treat all responses for a specific location the same.
* [ ] The RFC mentions that the cache must also purge entries based on the Location or Content-Location headers, so we might need to implement that.
:+1: I guess it should be better to use a blacklist of the safe HTTP methods instead of hardcoding the unsafe ones that should trigger the invalidation.