Add HTTP OPTIONS support

Fixes CAA-56
validate_mbid: correct the docstring
Make sure to raise NotImplemented on all HTTP/1.0 OPTIONS requests

Previously, requesting, for example "/*foo" with an OPTIONS request
would return 400 (or 200 if the entity was valid). This wasn't caught by
the test because it tested on a path of "/*" - the only one correctly
returning 501 for OPTIONS requests with HTTP/1.0
Merge pull request #5 from mineo/caa-56-pull

Implement HTTP OPTIONS support