Pan mode

This allows the user to move around the map without being pulled back to the center every time a new GPS fix comes in.  They can turn this off by touching the "Refresh" menu item.  Will move this later to a visible button similar to Google Maps. 

It's a bit convoluted, as the only reliable way I could find for OSMDroid to report that a user-initiated swipe has happened was to use an Overlay and listen for touch events.  When one happens, call back to the creator to store that it was a user initiated move.