Change UI to Extend Observable

- Prepare to pass the code to LoginManager through the use
of observer-observable pattern
- Remove unneeded methods
Implement Observer methods in LoginManager

- Change flow of execution in LoginManager
- Delete unneeded lines in TaskCommander
Merge branch 'master' of github.com:cs2103aug2014-f11-1j/TaskCommander
Add Placeholder For Sync Errors