Linux support

I wanted to try this on my machine (linux) so I added support for it.

I had to rearrange a bunch of code. I think it's compatible with OSX and Linux (tested on Ubuntu 14.04) now, but I don't have an OSX machine to make sure it still works there. 

All the system calls are now wrapped within a subclassed Adapter (ie OSXAdapter and LinuxAdapter), which is automatically chosen at initialization. It still has the same dependencies - imagemagick and ffmpeg - and the Linux adapter doesn't use the included video capture tool (videosnap).

Let me know if there are any problems.