Patch to allow unit tests to test for exceptions through a 'with' 
statement. Resulting (failing) test would look like this:

  import unittest

  class T(unittest.TestCase):
     def runTest(self):
        with self.assertRaises(KeyError):
           raise ValueError

This saves having to put exception raising tests into a try/except 
block, which is cool. And by cool I mean totally sweet.

I like this change, since assertRaises can be a bit messy when passing it 
a local function.
 
I'd suggest modifying the patch such that the AssertRaisesContext class is 
also used when callableObj is provided, which would eliminate the 
duplication of the code that checks for exceptions.

I've committed an improved patch, with tests and doc, in r67979 and
r67981. Thanks!