Add parallelism

Hey there, here's my behave fork implementing parallelization. Hope you like it ^_^; I also included tests for the parallel functionality. After this, I plan to add JUnit output; then try to merge some pep8 work done by a co-worker.

Oh, please cherry-pick the README.md out of your merge if you so choose to merge this stuff 

EDIT: I'm not clear on why TravisCI failed my PR. I can run the tests successfully'ish on my machine with the 
output:


    Failing scenarios:
      features/formatter.json.feature:234  Use JSON formatter with feature and one scenario with steps

    42 features passed, 1 failed, 0 skipped
    244 scenarios passed, 1 failed, 0 skipped
    892 steps passed, 1 failed, 0 skipped, 0 undefined



EDIT2: Oh, the new args I added in configuration.py need the right attributes. Okay. After that I'll try and squash these commits so it's easier for you to read.