if-modified-since header support

[tsloughter]

If the header if-modified-since exists in the request and the file has not been modified since that time a 304 should be returned.

[yurrriq]

I'd love this facility in elli proper a generic middleware. I've been doing it manually for data looked up in mnesia.

[yurrriq]

I've started working on a generic implementation in elli_cache.