plugins: make Plugins class, fix some bugs in the importing process and flesh out TestPlugins.test_initialise
enable basic implementation of plugins:
 * add Plugins.run method to run the correct methods from each plugin on an
   object
 * add smoke test for scraper.nugget_create
 * add last_nugget variable to User for testing purposes
 * update scraper methods to take nugget as argument
change User.just_added to > User.last_nugget