Made bike checkout popup display real station info

- Changed Station class implementation to extend OverlayItem so it can be added directly to the map along with relevant Station info.
- Updated MainActivity to use new Station class.
- Implemented REST call to get additional station info (like station ID number and number available bikes) to display in the popup box when tapping a station pin.
I think this is supposed to be indented. If indenting will push everything off, then I won't worry about it.