Fixed sorting(based on priority) bug

Fixed a bug which was related to sorting.
Currently, we are storing priority as a string/text in SQLite and I was
having an issue while sorting the todos based on priority.

The way I fixed it, I convert priority into integer and sort on the
bases on integer value. (0=Hight., 1=Med, 3=Low).

I think, it has little overhead to perform sorting right after reading
all todos from database. It can be resolved by storing the priority as
integer. Also, it make our TodoItem class more readable. Currenlty,
TodoItem has two overloaded getItemPriority() and getItemPriorityInt()
methods which are but confusing.

If you have any other good solution, please pass in my way… Thanks!