Stab at high complexity run method

Woah the preview still showed my working comments... sorry for the empty message before, here I go again:

The run method of Turnip::RSpec had a [rather high complexity](https://codeclimate.com/github/jnicklas/turnip/Turnip::RSpec) - this pull request is meant to solve this.

I extracted 2 methods of the 2 distinct main tasks this method achieves. Took me quite some time to see where to place the code but yeah it worked out in the end and I learned a lot about RSpec :-)

This is meant for feedback, as I like feedback in general. More specifically about the naming of the methods I created and the name of the module with which I extend ExampleGroup since this is still totally work in progress. Suggestions for this and of course overall suggestions very welcome.

Thanks for turnip + cheers