Added bookmark support for multiple Rokus/frontends

Users now have the ability to pause on one Roku/frontend and resume on
another Roku/frontend. Note that the Roku will automatically save the
bookmarks every 30 seconds, however, users will need to manually save
the bookmarks.

There are some additional notes about calcuating the bookmarks in the
README file.
Removed the need for $MythRokuDir and $mythtvdata global varibles
Updates made for issue #10.