Allow Celerity to return "304 Not Modified" responses

Summary:
We always return HTTP 200 right now and don't send a "Last-Modified" header, so
browsers download more data then necessary if you sit on a page mashing reload
(for example).

Test Plan:
Used Charles to verify HTTP response codes from 400, 404 and 304 responses.

Mashed reload a bunch and saw that the server sent back 304s.

Changed the resource hash seed and saw 200s, then 304s on reload.

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran
CC: bmaurer, aran, tuomaspelkonen
Differential Revision: 253