Fixed Plex notifications on PHT (preview)

Re-based onto develop, so needed to make a new pull request.

This fixes notifications for Plex Home Theatre (preview release) by supporting the Plex JSON API.

It requests /clients on the Plex Media Server to find the address and API type
(HTTP vs JSON) of the client names listed.

Plex client names are just the name of the computer as defined in the control panel.

I've confirmed it works with:

 - Plex Media Center 0.9.4
 - Plex Home Theatre (preview) 0.9.9.0

Closes issue #1908