Fix talons.helpers.import_function always returning None

In talons.helpers.import_function, the found function is not returned
and it returns None instead, causing TypeError when external.Authenticator
is used.
Add test to ensure talons.helpers.import_function returns a callable
Merge pull request #16 from sebasmagri/talons-helpers-doesnt-return-fn

Fix talons.helpers.import_function always returning None