Added login screen

Working on authentication process; password isn't being accepted
correctly...
Added separate PIN and password login screens
Added separate PIN and password login screens
Added functionality to retrieve texts from internal database, now displayed on ThreadsActivity

Now working on organizing these messages into conversations by sender.
Added functionality to retrieve texts from internal database, continued

Had to split into multiple commits.
Added dedicated action bar to threads screen, includes settings link

Also added on-click listeners to list items in the messages list. Will
later implement these to open detailed conversations for individual
addresses. Fixed a bug where the onResume method for the ThreadsActivity
was causing a crash; added the super.onResume() call to fix this.
Create README.md
Modified threads screen to only show most recent message from each contact.

Now working on querying phone database to replace contact numbers with
their display names.
Revamped threads screen. Now, clicking on a conversation takes you to the message center for that recipient.