Preparations for User-to-User Push Notifications

LoginActivity and SignUpActivity now both update the installation with
the current user (This allows for the notifications to know who the
current user is. This is required in case the user logs in as user1,
logs out and in again as user2. user2 will still receive notifications
from user1!
Also added KEY_USER_ID to ParseConstants.

RibbitApplication.java method:
public static void updateParseInstallation(ParseUser user) {
ParseInstallation installation =
ParseInstallation.getCurrentInstallation();
installation.put(ParseConstants.KEY_USER_ID, user.getObjectId());
installation.saveInBackground();
}