Added ./manage.py get_user_events custom command

To find user-specific events based on a user's nickname or IP address, as well as either a error group id and/or event id (or neither), try something like one of these:

    ./manage.py get_user_events --username=loisaidasam
    ./manage.py get_user_events --username=loisaidasam --group=1234
    ./manage.py get_user_events --username=loisaidasam --group=1234 --event=90210
    ./manage.py get_user_events --ip=192.168.0.27 --group=1234