Add entry in Settings.cfg to specify a IP address (temp fix of #84 & #87)

To temporarily Address the IP address issue for VPN Users, I added an Entry to Settings.cfg to specify an IP address for the PlexConnect DNS Server to be used for intercepting the Trailers Call from ATV. THe default is 'xxx.xxx.xxx.xxx' and can be changed to any IPv4 address. This address should match the IP address entered into the ATV DNS address field.

With the second commit I attempted to implement iBaa's suggestions for and enable/disable autodetect IP setting instead of relying on the xxx.xxx.xxx.xxx default, and to make the names of the settings more intuitive.