Adding MyPlex Support - based on fspijkerman's pull request #67

I've implemented a myplex settings page, and per-ATV storage of the myplex UUID.

To use it, go into settings->myplex and select log in. Enter username and password.
It validates authentication and stores the UUID.

Once the UUID is stored, the rest of the code works similarly to fspijkerman's pull request, except that I had to modify some of the functions to account for the fact that the UUID is stored in ATVSettings, not global settings.

I would love some input on making this better - I'm pretty new to Python.