Merge pull request #12 from Kami/improve_exceptions

Attach more information to the FFMpegConvertError errror object
Merge pull request #13 from Kami/update_ffmpeg_options

Update ffmpeg options
Merge branch 'master' into develop
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 branch 'parse_metadata' into develop