Various bug fixes
fix notification not working properly if groupMainActivity was not on top.
fix buttons state in groupsActivity when the user is asked for a password when comming from an autojoin.
small UI improvements, including a descent NFCManagerActivity.
Logging out when in a group is now disabled, the user is asked to leave his group first.
User experience improved when sharing a group using NFC. If the user is not already logged in, we ask him to log in then the app continues its autojoin process.
Merge branch 'NFC' of https://github.com/lca4/unison-android into NFC