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

# Problem

Current version doesn't support parsing video and audio stream metadata. Video and audio metadata can be very useful, because it can contain different information such as video orientation, etc.

# Proposed solution

This pull request adds support for parsing optional metadata which can be supplied with each audio and video stream. It also includes tests for this new functionality.

# Other changes in this pull request

1. Fixes a bunch of tests which were broken as part of #11.
2. Fixes a bug with specifying `-aspect` option even if both, source width and height weren't specified. This bug was introduced as part of #11.
3. To prevent issues like this from happening in the future (regressions), I'm working on hooking up tests with Travis CI. I will open a separate PR once it's ready and working.

All the tests pass now

```bash
$ python setup.py test
running test
......
----------------------------------------------------------------------
Ran 6 tests in 5.349s

OK
```