rewriting admin_validation tests and admin checks
split BaseModelAdminChecks from BaseModelAdmin
rewriting admin checks
moved BaseModelAdminChecks to checks.py
rewriting admin checks
rewriting admin checks + split [Inline]ModelAdminChecks from [Inline]ModelAdmin
rewriting admin checks + deleted commented code
deleted contrib/admin/validation.py
rewritting modeladmin tests
changed architecture: using composition instead of mixins
made FkNameCheckTests.test_missing_field green
solved problem with 'exclude' checks
made all tests green
deleted 'can_fail' argument from _get_foreign_key
made two inline_formsets tests green
implemented __str__ for ModelAdmin
renamed admin_validation -> admin_checks
validation -> checks in admin_checks
small clean up
validation -> checks in modeladmin