Introduced SmartTodoUser as a subclass of ParseUser.

After reviewing and making any other changes to the new SmartTodoUser
class, we should change all the references to the current User class to
this new class. Perhaps, we can do things in the following order:

1- Temporarily, remove this SmartTodoUser.java from the current
codebase.
2- Refactor the project by renaming the User class to SmartTodoUser
class.
3- Replace the old code of the just refactored SmartTodo.java with the
new code set aside in step 1.
4- Make other changes in the codebase to use the new SmartTodoUser class
properly.