Make the hipchat server configurable

The default value will still be api.hipchat.com to support what I
suspect to be the majority of users. But for those of us trying out
the new on-premise HipChat server we can also now use it.

Closes #7
Update README
[maven-release-plugin] prepare release hipchat-0.1.7
[maven-release-plugin] prepare for next development iteration