check export_queryset attribute on model.

Allow developers to filter object by user or add filtering options via GET/POST attrs (getting export from an other page)
Perf fix on a multi-column export edit in admin.

not tested yet
get performance fix for dj1.6 to work on dj<1.6
add BOM unicode to test excel ability to handle it
Update README.rst

Add a basic documentation and example of the model method to filter data.
README.rst example readability improvement.

Added the user related class (client).
Merge branch 'master' of https://github.com/magopian/django-data-exports
fix tox file flake8 migrations exclusion
Fix django deprecation  with empty exclude list.
Merge pull request #7 from Christophe31/master

Small update I needed (perf + queryset filtering)