http-service: Document pytest fixture
http-service: Add "/v1/refs/<shortname>" endpoint

It seems more RESTful this way, for the stb-tester Web UI to ask "what's
branch X's current head sha?" than for it to ask for all branches and
parse the data in the front-end.

This way, it's the back-end's responsibility to ensure that a shortname
like "master" is translated to a sha the same way that git would (for
example, once we add support for tags, git will favour a tag over a
branch with the same name).
http-service: Allow Cross-Domain requests from localhost, for testing

For dev-testing I'm running the stb-tester Web UI on localhost:8000, and
the http-service on localhost:8001. I need to enable CORS[1] so that the
Web UI can access the back-end API.

Once I have a better test setup with a reverse proxy we may be able to
get rid of this. On the other hand, it's limited to requests from
localhost so I don't think it will hurt to keep it.

crossdomain.py is copied from http://flask.pocoo.org/snippets/56/
verbatim, plus formatting changes to satisfy `make check`.

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS
http-service: Fix typo

For consistency with the HTTP route and with `TestRunner.run_tests`.