made changes to models

- added getters and setters
- implementing Serializable
- added some static methods to do conversions, get current user
added toString method
adding Operation enum to specify the reason why we came onto calendar page

- reasons could be:
ADD_NEW_EVENT, RESPOND_TO_INVITE, EDIT_CREATED_EVENT,
EDIT_RESPONDED_EVENT
Added initial code for CalendarActivity

- using square's custom calendar:
https://github.com/square/android-times-square. But this is just a
placeholder for now, will have to use a different calendarView.
- Views are in place