At least one broad functional test

[peterbe]

I suggest we use popcoder/tests/test_main.py to make one full run actually using ffmpeg. Put a something.json file in popcoder/tests/ and kick it off from a unittest class in test_main.py. The test can't obviously look at the outputted final video file but it should at least not error.

The sample .json file should do all supported operations. It might also be something we can re-use from setup.py for developers. E.g. "To test that it works, run python setup.py sampleencoding and then open and look at foo.webm" or something like that.