Adds support for using a personal HipChat server

HipChat Server currently in beta: https://www.hipchat.com/server
By setting the WILL_HIPCHAT_SERVER variable to the hostname of your server, Will can now use personal HipChat servers.
Don't use string adds
Address pull request comments from @skoczen
Merge pull request #46 from mrgrue/support-alternate-server

Support alternate server