Get rid of chai and other dependencies in favor of must

Must is a much nicer assertion library than chai. It's quite similar, but
has the benefit of not using accessor magic for assertions, making it less
likely you'll write tests that come back to bite you.

This commit ports all tests to use must instead of chai.
Add npm-debug.log to .gitignore
Use mstade/js-must until moll/js-must#18 gets merged
Use correct branch of mstade/js-must
Merge pull request #22 from mstade/must

Get rid of chai and other dependencies in favor of must