Add support for ETags and the If-Match headers.

Adding support for ETags and If-Match headers. On the client side, we need to
know if the entity that we have requested from poky is the version that we
expect. If the client makes a GET request under /kv that contains the If-Match
header then value supplied will be compared with the current modified_at
timestamp. If they are equal the GET will be processed as usual. If they are
not equal the request will be rejected with a 412 status code.
Updating the README with If-Match.
Don't cache /status requests.
Varnish support for If-Match.
Adding 412 logic for the if-match not found case.

/cc @loganlinn
Merge pull request #13 from drsnyder/if-match

Adding support for If-Match