Improve error message when fields don't match dataset.

Currently if you upload a file that doesn't include a required column, you
just get an error like "KeyError: 'id'". This patch improves the error message
so that the problem is easier to understand and solve.
Commit a blind fix for Python 3.3 errors.

https://travis-ci.org/bmihelac/django-import-export/jobs/28887412
Another blind fix for python 2/3 issues.
Merge pull request #109 from aptivate/fix_missing_field_error_message

Improve error message when fields don't match dataset.