Add a context manager for verification

[ivancrneto]

Idea for a verification context manager:

class Verification(object):
    def __init__(self, m):
        self.m = m

    def __enter__(self):
        self.m.ReplayAll()

    def __exit__(self, exc_type, exc_value, traceback):
        if exc_type is None and exc_value is None and traceback is None:
            self.m.VerifyAll()
``

This would allow one to say:

def test_foo(self):
    with self.mox.Verification():
        # …

This nicely delineates the code under test from the setup code.