feat(api) expose components to the outside world for testing

Exposes constructor functions for models by placing them within the
InboxAPI namespace.  Exposes the InboxAPI namespace to Angular via
$inboxProvider.  This allows tests to access the methods on these
objects.  A specific example is using INDraft.attachments() to ensure
that attachments have been added correctly and reflect the correct ID.
Merge pull request #44 from zipcode/zip-access

feat(api) expose components to the outside world for testing