[MRG + 1] [TST] (half-cosmetic) use less nose.tools import to simplify future...

[MRG + 1] [TST] (half-cosmetic) use less nose.tools import to simplify future transition to py.test (#7384)

* use less nose.tools import to simplify future transition to activly developing test suites/runners

* assert_equal -> assert_array_equal in test_feature_hasher_pairs_with_string_values

and one missed ImportError that should be replaced with AttributeError

* test for py2.6 compat with except AttributeError

* fix importing of SkipTest

* force using nose in python2.6 for now

* there was no assert_dict_equal in py2.6. but we can use assert_equal

although failed test will look a little bit ugly

* remove nose imports from doc/datasets