Allow listeners to be looked up by name

This does an Object.const_get to find a pubsubhub listener if the user
provides a string, allowing lazy evaluation of constants so as to not
cause any side effects of unnecessary class loading.

Specifically, this has caused us trouble twice - once in trying to
include one of our mailers in pubsubhub, which creates records as a side
effect, and once in the Rails 4 upgrade, which insists on having
assets:precompile load the environment (and we're hoping to get it to do
that without connecting to the database)

Change-Id: I0b2572d43c520c2a320d95492cb1f69f29199cea
Reviewed-on: http://gerrit.causes.com/35711
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Tom Dooner <tom.dooner@causes.com>
Release version 0.0.3

Change-Id: Id696f1af030f60f7a02ed15fc9cce0bc7e9d3a95
Reviewed-on: http://gerrit.causes.com/35713
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Tom Dooner <tom.dooner@causes.com>