Many edits to the app, mostly to fix bugs, with some style edits.

Among the bugs fixed:
* Due dates now don't show when they're not supposed to.
* The task list refreshes when necessary to reload.
* Added a refresh button to the task list screen.
  (It's on the activity title on the top right.)
* Added spacing in front of the priority and due date on each task item.
* Edited the "About" Toast.