Features/aac

This branch fixes a bug and adds support for AAC Stream Key formats. 

AAC stream keys now have the format (codec)-(samplerate)-(profile index)-(number of channels). Profile index is not useful at the moment for encoding until we want HE-AAC support, but it does let us know what kind of AAC stream we are dealing with from Samcast. 

I would like to spend some time refactoring this to make it easier to understand and to make it easier to extend support for future audio encoding formats like wav and High Efficiency AAC. 

Note that this branch will only work with the forked version of FFmpeg on SCPR: https://github.com/SCPR/FFmpeg
I plan on submitting a PR to FFmpeg to support an option for changing MPEG versions so we no longer have to maintain a forked repo.