Add 'beforeEach' and 'afterEach'

[Ryan Mason-Davies]

'beforeEach' is a hook that acts the same as the SenTestCase -setUp method: it is invoked directly before an example is invoked, after all context implementations have been invoked.

'afterEach' is a hook that acts the same as the SenTestCase -tearDown method: it is invoked directly after an example is invoked.

Both can be nested. Before a context implementation is invoked, its beforeEach hook should be.