Promises tests and improvements

Hey,

as you requested in #366 I had a look at your promises implementation.

There's a test suite for Promises/A+ which I included as part of the `grunt test` command. However, for development it's probably better to focus on a single paragraph of the spec using the CLI like this: `node_modules/.bin/promises-aplus-tests test/promises-aplus-adaptor.js --reporter spec --grep 2.2.6`

I fixed some issues, but there's still a ton of failing tests. Unfortunately, the tests are sometimes hard to understand...

I'm opening this PR right now because I probably won't have any more time to do anything here this week and want to prevent duplicated efforts.

-- Luchs