Prevent infinite post processing with Keep Original Files enabled and ...

Issue 1538 - 'Keep Original Files' + 'Scan and Process' = infinite processing/notification loop
https://code.google.com/p/sickbeard/issues/detail?id=1538

If the Keep Original Files option is used and when the post processor copies a found video to the new location, this change will also create an empty file with the same name as the video - but ending in .processed.

In turn, when ProcessTV.processDir() checks videos for processing it will ignore the videos already having that empty helper file, thus not post process that file again. 

This change is very important for users running a black hole torrent setup, with a torrent provider which requires them to keep seeding for good ratios. Without it the post process will process all the found videos again and again, every run.