Audio thumbnailing & spectrograms, media plugins use sniffing

* Added extlib/freesound/audioprocessing.py
* config_spec
  * Added create_spectrogram setting
  * Added media:medium and media:thumb max_{width,height} settings
* Added sniffing logic to
  - audio.processing:sniff_handler
  - video.processing:sniff_handler
* Changed audio.processing:sniff_handler logic
* Added audio thumbnailing functionality to audio.processing
  (works only with create_spectrogram enabled)
* Refractored contexts in audio.processing
* Added audio.transcoders:AudioThumbnailer
  Used for creating spectrograms and spectrogram thumbnails -
  Wadsworth's Constant, we meet again :)
* audio.transcoders:AudioTranscoder
  - Added mux_string kwarg
  - Delete self.pipeline on self.halt()
* Changed str.format formatting in image.processing:sniff_handler
  Had {1} without an {0}, changed to {0}
* Refractored VideoTranscoder to use transcode() for transcoding instead
  of __init__()
* Added discover() method to video.transcoders:VideoTranscoder
* Added spectrogram display to media_displays/audio.html
* Updated test_submission to reflect changes in media plugin delegation

mediagoblin/tests/test_submission.py

@@ -231,7 +231,8 @@ class TestSubmission:
 
         context = template.TEMPLATE_TEST_CONTEXT['mediagoblin/submit/start.html']
         form = context['submit_form']
-        assert re.match(r'^Could not extract any file extension from ".*?"$', str(form.file.errors[0]))
+        assert 'Sorry, I don\'t support that file type :(' == \
+            str(form.file.errors[0])
         assert len(form.file.errors) == 1
 
         # NOTE: The following 2 tests will ultimately fail, but they