Start making response generation a thing.

So far this is a bit of a spike. Tests are inadequate.
Complete the set of responses for implemented operations.
Started specing up the various responses.
Cleaned up testing of error responses.
DRY testing common headers in responses.
Small fixes to response generation.
Return the right resource for NoSuchKey errors on GET Object requests.
Make long lines shorter.
Merge pull request #2 from envato/better-responses

Better responses to S3 requests.