Created simple Sign Up activity

Allows the user to sign up for a new account. No actual functionality
yet.
Changed logic for logging in

When you are not logged in, LoginActivity is displayed when you
first open the app. If you are logged in, however, ConversationsActivity
is displayed instead. Nifty!
You can now logout from the ConversationsActivity

When you logout, a setting is toggled that puts you back to the
LoginActivity.
Merge branch 'signup' into develop