Refactor test suite

[bendrucker]

Wanted to do this as part of the process of diagnosing bookshelf/bookshelf#310. Changes include:

• Swapping Q for Bluebird to take advantage of Bluebird's onPossiblyUnhandledRejection feature
• Updating to Mocha 1.8.x which has native promise support — you can return a promise to make a test async rather than needing to call the test callback
• Using Chai and Sinon (+ plugins) for more expressive and specific assertions
• General cleanup of tests so that each test covers more specific functionality

[tgriesser]

Thanks.

[bendrucker]

Closes #2