Allow you to specify the hipchat sender on the commandline.

Summary: This was an oversight it wasn't added before.

Test Plan:
Manually ran
   echo 'hi' | env PYTHONPATH=. third_party/alertlib-khansrc/alert.py --hipchat-sender='TestiGator' --hipchat='HipChat Tests'
and saw the message show up on hipchat with TestiGator as the sender.

Reviewers:

Subscribers: