Modify probe method to also parse video and audio metadata.

Now MediaStreamInfo object also contains 'metadata' attribute which is
dictionary and contains video or audio stream metadata.
Add a test case for metadata parsing.
Update a bunch of tests which were broken as part of #11 and were never fixed.

Also fix a bug with always passing -aspect option to ffmpeg.
Merge pull request #14 from Kami/parse_metadata

Parse video and audio stream metadata and fix a bunch of broken tests