Screenshots

Hey,
via this PR I'm adding the possibility to include screenshots - via the ```--screenshots``` arg - into the HTML report (useful for UI testing using KIF for instance). 
![screenshotfeature](https://cloud.githubusercontent.com/assets/526374/3351531/521e10bc-fa10-11e3-92d4-aa9d18172b70.gif)

BDD-like tests are provided. 

Known limitations:
- the image files should be in the same directory of the HTML report (e.g. build/reports/)
- matching is performed via the filename (e.g. "RACCommandSpec, line 80, hello xcpretty.png" matches the RACCommandSpec suite)

I would have liked to perform automatic gif generation but I didn't want to add the rmagick dependency to xcpretty in order to keep the lightweightness of the project.

I'm open to all sort of discussion and modification request.
Best,
@viteinfinite