Change data flow from Controller to UI

- Change return type of executeCommand from Feedback to String e.g.
"added: "Date" taskName"
- Add getDisplayedTasks methods which is called by the UI to refresh the
table; the display command is only used to filter or restrict the tasks
viewed by the table. This settings are saved in the Controller, so that
they remain until the user changes them.
Merge branch 'master' of
https://github.com/cs2103aug2014-f11-1j/TaskCommander.git