0.8dev: Isolate import of `unittest2` module to a single location.

The comparision `sys.version < (2, 7)`  in `tests.core` was incorrect (the correct comparision is `sys.version_info < (2, 7)`). To simplify things, we import `unittest2 as unittest` in `tests.__init__` and import unittest from `tests` everywhere else.

git-svn-id: https://svn.apache.org/repos/asf/bloodhound/trunk@1536964 13f79535-47bb-0310-9956-ffa450edef68