ENH: Add new specialized asserts for tests
Work on #6175. Add versions of asserts (assertIs, assertIsNone, assertIn, plus complements) that were added in 2.7

This is meant to be API-compatible versions of some assertions that were added in 2.7 (http://docs.python.org/2/library/unittest.html), that would be useful for cleaning up #6175.

I am open to suggestions; putting this out as an experiment. Many of the current assert_'s could be specialized to the new asserts that were added in 2.7, but since pandas currently supports 2.6+, I figured it would be easier to add (versions of) the new asserts to the testing module.
 @bwignall that's all fine....compat is good!
