Added a setting for Mac address for each Plex Server

This is the first part of Wake-on-LAN (WOL) functionality for Plex on
Roku.

All this commit really does is add a setting to set the MAC Address of a
Plex Media Server that's being used.  The MAC Address is required as
part of the WOL protocol.  This should have been relatively
straightforward, but there were a number of challenges, namely:

* The Roku doesn't seem to have the ability to manage ARP packets via
BrightScript or offer any functions to do an ARP.  Otherwise, we could
have scanned the MAC Address by IP, and not even required the user to do
anything for WOL.
* The list of Plex Media Servers is stored in a horrible comma delimited
string, which would have been problematic to change to maintain
backwards compatibility.  I therefore created a new Registry entry,
"serverData" which is a tidy JSON storage for server settings in future.
This of course required the addition of a JSON encoding library, which
has a compatible license.
* The settings menu seems to have been designed with absolutely no
hierarchy in mind.  I've noticed ljunkie has struggled with this issue
as well - there should be some considerations about trying to come up
with a more flexible design for this in future.

To add a MAC Address to your server, simply Go into "Preferences" ->
"Manage Plex Media Servers" -> "Edit <server name>" -> "Edit WOL Mac
address".   This should be in the format of xx:xx:xx:xx:xx:xx, or the
value won't be accepted.