Append custom message to notification, configurable per Project (includes var evaluation).

I reviewed the existing pull requires and non of them really accomplished what I was trying to do.  The Mustache templates change is much more than I need and the 'Add custom message variable' is too simple (note configurable per project and doesn't eval variables).  Here is what I was trying to accomplish:

1) Give the option to append a custom message to messages sent to hipchat.
2) If the custom references variables (i.e. ${var_name}), evaluate them.
3) The custom message need to be configurable on a per-project basis.

The PR accomplishes this.  I see this as being a nice option along side the other PR currently being considered.

Please let me know what you think.  Thanks!