Added DB functionality for login page

The login page now takes data from the user and sends it to the PHP
script, receives the results of the SELECT statement back in the form of
a JSONArray, and then parses it, checks it against the input to see if
the strings are identical for username and password, and if so, sends
the user to the next page.

TODO: Add custom error message for bad username/password combo in the
form of red text TextView