fix name of template arg
move importer variant enumeration out of main importer function
make importer classes yield task data; have importer do the adding
remove extraneous references to app from internals of importers
remove use of app in importers
change error message explicitly referenced from test code(!?)
Merge pull request #475 from PyBossa/generalise-importers6

Generalise importers - tidy up class interface
Merge branch 'master' into culttag-redux
update to reflect interface changes