Codifying a pid based job

Given that we are passing PIDs into jobs, we shouldn't assume that the
received PID will be for a Generic File. By using casting a base find
into the class we can more easily send other things through the various
jobs.

Conflicts:
	lib/sufia/jobs/characterize_job.rb
	lib/sufia/jobs/unzip_job.rb
	sufia-models/lib/sufia/models/jobs/import_url_job.rb
	sufia-models/lib/sufia/models/jobs/transcode_audio_job.rb
	sufia-models/lib/sufia/models/jobs/transcode_video_job.rb