dumpscript: more 3 hooks to import_helper.py and we now use a class for that

import_helper.py now should contain a class ImportHelper()
This class will extend BasicImportHelper so it can overwride
the following methods ( import hooks ):

pre_import(self):
run_import(self, import_data):
post_import(self):
locate_similar(self, current_object, search_data):
locate_object(self, ... )
save_or_locate(self, the_obj):

These changes make DumpScript very flexible.
Now we can, for example, use transactions while importing
by just reimplementing run_import.