Provide a stand-alone context manager

Hi, 

I thought it would be useful to provide this context manager in a way that does not require subclassing from TestCase.

I like using py.test for my tests, and I dislike combining py.test with the x-unit test case styles. django-with-asserts seems awesome to me, and in looking at the code it seems the only usage being given to the base TestCase class is a call to assertEquals.