Display output URLs as hyperlinks in Hipchat

By default the output URLs are displayed in Hipchat as plain text and must be
copy pasted into a browser. This change allows Hipchat to transform them into
clickable hyperlinks (by default, you can turn it off).

*Tech Notes*
Hipchat now allows API messages to be processed similar to user messages by
passing `message_format: text" in the API call.

Relevant Hipchat Blog Post
http://blog.hipchat.com/2012/07/10/a-simple-but-powerful-change-to-our-messaging-api/

Relevant Hipchat API Docs
https://www.hipchat.com/docs/api/method/rooms/message
Improve readability of patch
Merge pull request #118 from mal/patch-1

Display output URLs as hyperlinks in Hipchat