instantiate sender objects specific to each app environent

We don't want to be fakeweb-bing or stubbing honeybadger when we test custom notifications to honeybadger that we emit in our rake tasks and job queues.  So we established a sender object for each of the three base environments

* production: Honeybadger::Sender
* test: Honeybadger::TestSender
* development Honeybadger::NoopSender