Cleanup Startup of TvShowNextAired

See: http://forum.xbmc.org/showthread.php?tid=193682&pid=1696392#pid1696392
Check that TvShowNextAired is installed

To remove the mandatory install of script we need to check if installed
Remove TvShowNextAired Mandatory Install
Disable TvGuide if TvShowNextAired Missing
Add Skin Settings Option for TvShowNextAired

Flip TvTunes over to enabled by default as well for consistency
Reverse Watched Overlays

Enabled by default now, user must disable
Merge pull request #58 from bumpaneer/Gotham

Remove TvShowNextAired as Mandatory