Add HipChat notification helper

This adds a new helper for sending notifications to HipChat rooms
Fair point, that should work too. Are there plans to update FAKE to .NET 4.5? If so, [`WebUtility.UrlEncode()`](http://msdn.microsoft.com/en-us/library/system.net.webutility.urlencode(v=vs.110).aspx) would work as well.