Allow namespaced listeners

Trigger was using Object.get_const which doesn't support namespaced
objects. Instead of adding Rails dependencies for its constantize
method we chose to just add a simple constantize method instead.

Change-Id: I80450635126b30d9dca71694aaaa3dd12244d73f
Reviewed-on: http://gerrit.causes.com/38356
Reviewed-by: Brian Mason <brian.mason@causes.com>
Tested-by: Ben Wiseley <ben.wiseley@causes.com>