Renamed sessionId to userID and class extends Response.

Sarah, you will have to update the following two lines in LoginActivity:

Log.v("MainActivity", "sessionId: " + loginResponse.getSessionId());
sessionId = loginResponse.getSessionId();

.. as server is sending userId of the user we just tried to log in. This user id will be be sent for subsequent calls to the server for say adding a tweet or looking up user profile, etc.