Add support to ParallelTests

Hi,

since i needed the feature from #150, i've recreaded the patch from @rafaelp on top of your current master.
It should be mergeable now.
I hope to not annoy @rafaelp with that. I/we know that you created the patch in the first place (Thank you again for that :yum: )

Here is his original message:

> I've added a option to run specs using parallel_rspec instead of rspec command.
> 
> There are more people looking for it, as we can see at http://stackoverflow.com/questions/13470493/is-it-possible-use-guard-in-combination-with-parallel-tests