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.
Add travis config and modify tests so they work on travis.
Merge pull request #14 from Kami/parse_metadata

Parse video and audio stream metadata and fix a bunch of broken tests
Merge pull request #15 from Kami/add_travis_config

Add travis config and modify tests so they work on Travis CI
Add mp3 format to allow converting to mp3 files
Add test to convert to mp3
Log ffmpeg commands
Create multiple thumbnails in one ffmpeg call
Changable jpeg quality when creating thumbnail
Change DEFAULT_JPEG_QUALITY to 4, to have good quality with reasonable size
Merge pull request #16 from tahajahangir/add-mp3-format

Add mp3 format to allow converting to mp3 files
Merge pull request #18 from tahajahangir/improve_logging

Log ffmpeg commands
Grab stream bitrate in MediaFormatInfo
Grab stream bitrate in MediaStreamInfo
Stream bitrate should be None when it's N/A on ffprobe output
Add tests for MediaStreamInfo.bitrate
Merge pull request #17 from tahajahangir/grab-stream-bitrate

Grab stream bitrate
Assert thumbnails exist after call to `thumbnails`