Doc changes

Hello.
I've added a few more descriptive wordings to the README to make the doc a little clearer.
Also a few tweaks to the hipchat-post-receive script, specifically:
Using = instead of == for strict POSIX compliance. Without it my git push would have succeed but had a bunch of errors spewn in it about unexpected operator.
And also I changed the variable HIPCHAT_ROOM to HIPCHAT_ROOM_ID as it looks like the hipchat-cli expects a room_id and not a room name as the README seemed to suggest.

thanks,
-Mehryar