Added support to avconv, Added mediainfo util for quering video metadata

There is now support to avconv outof the box, I just can't find how to make travis do 2 different builds for testing.

Build without ffmpeg dependency:

```yaml
before_script:
  - sudo apt-get update --fix-missing
  - sudo apt-get install -y libav-tools libavcodec-extra-53
language: python
python:
  - "2.6"
  - "2.7"
  - "3.2"
  - "3.3"
script: python test/test.py
```

Any idea?