Test helper that asserts a given transition occurred

[markrwilliams]

Automat should have a test helper that asserts a given transition occurred after running a block of code.

It might go a little something like this:

with assertTransitioned(someMachineInstance, 
                        through=[SomeMachine.state1, SomeMachine.state2]):
    someMachineInstance.blah()
    someMachineInstance.otherblah()

[markrwilliams]

@tomprince suggested this after working on http://twistedmatrix.com/trac/ticket/8993. This could use #36