Adding support for If-Match

[drsnyder]

This pull request adds support for If-Match. The behavior is as follows:

On GET:

• If the If-Match header is not supplied return the content as usual.
• If the If-Match header is supplied, compare it to the RFC1123 formatted version of the modified_at timestamp. If they match return the content as usual. If the don't match, return a 412 (Precondition Failed).
• Include an ETag header in all GET responses.

/cc @loganlinn

[loganlinn]

LGTM aside from comment about returning 412 on miss