Add Amazon Linux support

* Added support for Amazon Linux platform. Tested on 2012.09 version.
Amazon Linux does not provide ffmpeg yum packages, so they have to be installed from source (ffmpeg and its dependencies). So I've added an LWRP provider that installs an ffmpeg module given its source url and compile flags. These modules are installed or not depending on the ffmpeg compile_flags (as before).

* Changed the ffmpeg git repo to the new one `git://source.ffmpeg.org/ffmpeg.git`

* Added cookbook `ffmpeg` name in metadata