FFmpeg Source + Unified FFmpeg CMake find module

Add obs-ffmpeg-source to obs-ffmpeg plugin.

Unifies the different FFmpeg find modules in to one to avoid having to repeat code all the time when you need to modify how the libraries are found.

I've tested the build on both OSX / Linux, but not windows as I don't have a windows dev box.

The FFmpeg find module was just for my sanity as I was replicating a lot of already-written code just to write a find module for libavdevice.

# Cool Applications
Loop some movie:
![screen shot 2014-09-16 at 2 30 11 pm](https://cloud.githubusercontent.com/assets/1236452/4294513/5b76472e-3de0-11e4-8fae-ac28c035d9a5.png)
Display webcam via `avfoundation` indev from libavdevice
![screen shot 2014-09-16 at 2 31 38 pm](https://cloud.githubusercontent.com/assets/1236452/4294538/9079588a-3de0-11e4-9e76-81ff14f788ee.png)