Refactor Episode#output to be a method

Episode#output used to be a stored property. However, given that it
never changed and was a precalculated value there was no need to store
it.

Additionally, added the Episode#screenshot method. This method is a
companion method to Episode#output. It provides the path the Episodes'
screenshot taken from FFmpeg.
Implement Resque worker to generate screenshots
Display screenshots
Change Guard RSpec settings