add wrapper functions for get and post
Stub of a LoginScreen
Initial stub for list activity
Integration of zxing qrcode library
Defined main app structure (Login - List - CheckIn - DetailsView)
Intial graphic
Added custom adapter
Changed confirm and cancel buttons layout
UX changed and simplified.
Multiple check in are now allowed without opening the camera each time
Add another model to handle to-be-paid checkins and a listFragment to manage them
Add support to swipe
Finished a first code refactoring.
All http requests are now handled by a single  IntentService instead of a bunch of AsyncTasks. The service is even in charge of the json data parsing