Acceptance tests

Finally! Guard has Cucumber/Aruba tests, so:
- bug reports can be made "reproducable" with Cucumber scenarios
- current specs can use mocking more aggressively (which REALLY would help with refactoring)
- I'll stop breaking basic functionality without knowing (e.g. `guard init`, `guard show`, etc.)
- complex/multistep scoping issues can now be reproduced!

Also, I'm using "in-process" mode of Aruba, which makes these acceptance tests really fast...

I'll add new *.features files as I go and as I discover new bugs.

Note: this also obsoletes fix #651 (since the features showed how broken 2.7 is right now...)