basic implementation for a cucumber command

closes #31

very basic implementation for a cucumber command. It works with a single feature but I had troubles running the whole suite. I'll investigate:

**first**
```
$ time ./bin/spring cucumber features/my.feature       ~/Projects/equis/backoffice  
Using the default profile...
(eval):1: warning: toplevel constant FactoryGirl referenced by Backyard::Adapter::FactoryGirl
..........

2 scenarios (2 passed)
9 steps (9 passed)
0m0.907s
./bin/spring cucumber features/my.feature  0,78s user 0,11s system 7% cpu 11,916 total
```

**second**
```
$ time ./bin/spring cucumber features/my.feature  
Using the default profile...
(eval):1: warning: toplevel constant FactoryGirl referenced by Backyard::Adapter::FactoryGirl
..........

2 scenarios (2 passed)
9 steps (9 passed)
0m0.927s
./bin/spring cucumber features/my.feature  0,78s user 0,11s system 25% cpu 3,462 total
```