Add failing tests for removing time-tuple

Before, documents were represented as `(date, dictionary)`. `date` is
superflous since it's in `dictionary["start_date"]`, and screws up the
plugin pipeline (#40).
Fix to remove superflous date
Add empty transformation plugin implementation
Eliminate nosetests star import
Tweak noop plugin implemenation
Add failing test for plugins
Run plugins after build_document_set