Merge pull request #113 from aptivate/add_column_name_to_clean_error_message

Add column name to ValueError from Field.clean()
Properly handle import/export of NullBooleanField
Updated changelog and added Andrea Cimino in authors file
Merge pull request #115 from andreacimino/master

Properly handle import/export of NullBooleanField
Note: Backward Incompatible Change
Merge branch 'master' of https://github.com/bmihelac/django-import-export