Added a "Don't Show Again" dialog class.

Replaced the previous AlertDialog which had a Don't show again checkbox
with this dialog class.
Added Welcome dialog.

Added Welcome dialog, ana associated variables for persistance.
Changed to calling dialogs on separate threads.

Changed to calling dialogs on separate threads.
New pattern using activity as a dialog.

New pattern using activity as a dialog.
Fixed welcome dialog and post dialog execution.

Fixed problems related to application moving to the background, esp.
while showing instructions from welcome screen.
Merge pull request #158 from mafiglio/welcome_screen

Welcome screen