FIX: #25 - Overriding queryset on export() fails when passed queryset has zero elements

Thanks to @photojosh for report and solution
Removing Resource method `set_instance_attr` as it is not called anywhere in the codebase and infact calling it fails as there is no `get_mapping()` method defined on Resource or any of its subclasses.
Merge pull request #33 from djm/feature/remove-set-instance-attr

Removing Resource method `set_instance_attr`
Add djm to authors
Update CONTRIBUTING.md

fix link
Update getting_started.rst
Use  instead of  for field dehydration, FIX #36
Update docs for admin integration
PEP8 fixes
Remove unused import
Add ``export`` method docstring
FIX #38 - Export button not visible when no add permission
Remove testing code added in 9aa49e898db4a00086eb099a5bc32f84c490eb32
Fix typos.
Merge pull request #42 from pjdelport/master

Fix typos.
Update AUTHORS
Merge branch 'fix-36-use-field-name-for-dehydration'
Mention fixing #23 in changelog
Prepare for release
Merge branch 'master' of github.com:bmihelac/django-import-export