Remove cucumber in favor of just RSpec and Cucumber

This also adds https://github.com/ngauthier/domino for defining DOM elements.

I tried to make `spec/features/keg_spec.rb` closely mimic the test in `features/keg.feature`. However, I think that this highlights testing with Cucumber can lead to tests that test lots of stuff instead of pin pointing specific tests.

It also cuts test suite time in half.

**master** `time be rake`
```
real    0m20.900s
user    0m17.979s
sys     0m1.885s
```

**remove-cucumber** `time be rake`
```
real    0m10.383s                                                                                                                                                                                   
user    0m8.388s
sys     0m1.214s
```

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