Remember Last Station Played

I had a use case in my home where I wanted to map the Pandora Addon to a remote button and not have to find my favorite station to resume play each time. 

I added a setting to keep track of the last station played. If that same station still exists the next time the addon is run it will resume playing on that same station when everything is done loading. If this is annoying for some people it could easily be set to look for a true/false "Play last station on startup" value as well. Let me know if this is something you want me to add as part of this pull request. 

I tried not to make it too "hackish" and I think I put the correct pieces where they would logically go in the code. Hopefully this is something others would find useful.