Added item prioritiy

Adds: items have now a value of priority that influences the order in
which the items are shown.
Adds: priority can be modified via contextual action bar.
Change: the create item dialog now have a spinner where the item
priority can be set.
Change: detail list view now have a small rectangle indicating the
item's priority.
Adds: contextual action bar has a new feature that let the user create
an event on the calendar app.