Added framework for bike checkout/in

MyItemizedOverlay class now keeps track of whether a bike is checked out
or in and displays different popup messages depending on its state.
Made bike checkout/in more visually distinct.
Implemented checking out a bike.
Implemented bike checkin.
Whitespace fix.
Removed unused imports.
Added popup dialogs for error cases.
Integrated checkout/checkin with UserSignIn.
Added warning that user_id no longer exists.
Merge pull request #10 from bikeshare/rest_checkinout

Implemented bike checkin and checkout