Support for avconv (aka Libav)

This is a starting point towards supporting ```avconv```. Ubuntu builds now have avconv more readily available than ffmpeg, especially the new OSes (14.04+).

This code works properly as-is. However, I would like to discuss the following: should we make it a separate processor (as I did here) or use some mechanism to detect whether or not ffmpeg is present and then try cascading to avconv if it's not present?

Simply put, the code in this commit is a copy of the ffmpeg processor with the following changes:
- ```ffmpeg``` became ```avconv```
- ```Ffmpeg``` became ```Avconv```
- ```ffprobe``` became ```avprobe```

and then in my Rails app I simply change the Paperclip processor to ```:avconv``` in the model.

Once we agree to the proper way to proceed then we can add some tests and bump the version, etc.

What do you think?