JS runner for True tests.

It works! In this pull request, True's own Sass tests are now run through Mocha. You can see them pass if you run `make test` (after having done an `npm install`). If you add some failing tests, you can see what it looks like when tests fail.

There's no way (that I could find) to write a plugin for Mocha so it will directly discover SCSS files (py.test could do that - maybe we should write this in Python instead :P). So you have to have a `test/test*.js` file in your project with just a couple lines of boilerplate for Mocha to find and run your True tests. You can see in `test/test_sass.js` what that looks like.

I think this PR could be merged as-is, but here are some things that could be done that I haven't yet:

- [ ] write usage docs
- [ ] add support for other JS test runners (any that use nested syntax similar to Mocha may already work)
- [ ] register `sass-true` in the public npm registry; for anyone using it via the JS runner, `npm` would be the most logical way to get it
- [ ] add a command-line runner similar to `true-cli` but that uses Mocha instead, so you just point it at a Sass file and go. One question here is whether we want true itself to depend on Mocha -- right now it doesn't (its only in the `devDependencies`, since its used to run true's own tests), and you could potentially use the test-runner integration I wrote with various different JS test runners (though I haven't yet tried it with any other than Mocha). Another option might be to provide separate packages like `sass-true-mocha` etc that pull in a dependency on a specific test runner and add conveniences like CLI scripts.

I'm sure you'll also have opinions on the output format for failed tests once you start using it, so that'll probably take some tweaking over time.