Improved code in Transcoder class.

Video files are now checked for corruption befoer being sent for post-processing by either SickBeard, NzbDrone, or CouchPotato.

Added ffmpeg_path option to Transcoder section allowing end-user to set where there ffmpeg is located.
Added in some missing code
Sending ffmpeg checks for executables to null
Missed another line of code, had to add
Corrected a conditional check mistake for testing corrupt videos
Detects ffmpeg and ffprobe at startup but will alert you to if there missing only when needed to be used so that we don't get a stupid error message at startup for no reason.