Add jshint to the tests (fix #6)

This is inspired by the test of the same name in browserid-local-verify
(https://github.com/mozilla/browserid-local-verify/blob/404bdcd158094aaaeb412b8eb0263243c79f8b92/tests/jshint.js) and enables all of the checks that pass.
Merge pull request #10 from fmarier/jshint

Add jshint to the tests (fix #6)