dataapi: _set_cache_for wrong agruments order bug fixed

This was series stupid mistake. Instead of creating key for cache get
method using only name argument, DataAPI.get._cache_set was using
value as a key
dataapi: _set_cache_for exists cache set to True as well

This makes sense since we're setting up "get" cache key a line above.
dataapi: unnecessary line in the end of file removed
dataapi: attrgetter usage removed - replaced with list comprehension
dataapi: set inconsistency with/without cache removed

DataAPI.set method used to return two different values weather cache
was activated or not.
tests: api & modelregister tests unregister_all usage removed

Usage of django_settings.models.register.unregister_all causes
problems in other testcases since all of them makes assumption that
default setting types are simply available (which is randomly not true
when you'll use unregister_all).
tests: proper assert_queries_count method added

it's now based on django's assertNumQueries assertion
tests: dataapi tests refactorisation and fixes

Previous version of assertion that was counting queries was not work
as expected. Also more test has been added and futher simplifications
has been applied.
tests: removing "n" helper (step 1) - TestCase with basic assertions created
tests: removing "n" helper (step 2) - test_modelregister cleanup
tests: removing "n" helper (step 3) - TestCase.assert_raises added
tests: removing "n" helper (step 4) - test_api cleanup
tests: removing "n" helper (step 5) - test_cache cleanup
tests: removing "n" helper (step 6) - test_data_cache cleanup
tests: removing "n" helper (step 7) - TestCase.assert_items_equal added
tests: removing "n" helper (step 8) - test_admin.tests cleanup
tests: removing "n" helper (step 9) - removed "n" helper
cleanups: unused imports removed, blank lines removed
tests: assert_items_equal own simple implementation provided

unittest for python 2.6 (which is still supported) doesn't have
assertItemsEqual method.