GPS displayed on MapActivity

This would close #484.

* I actually didn't like the "marker" so I left it out from 2ccb882, but I can revert to 6d058f5 if desired. Not too happy with the design, so give me your ideas.

* It right now uses GPS intents (minimizes codebase impact), which makes it behave... spottily at start, screen turn, etc. It might have to be re-implemented through the AIDL communication instead.

Comments welcome.