Adds various features to the login screen

In the login screen, there is now a "Quest Login" title, and
there are fields for the user to enter his or her Quest username
and Quest password. There is also a "Sign in" button, which the
user will click after filling out his or her credentials.
Finally, the theme for the entire app is changed to Holo.
Renames MainActivity class to LoginScreenActivity.

This has been done to reflect the fact that the activity's layout is
called login_screen_layout. Thus, to remain consistent and to provide
clarity, the MainActivity class has been renamed to LoginScreenActivity.