Adds support for Cedar-style +beforeEach and +afterEach class methods

I've found Cedar's global beforeEach and afterEach methods really useful. They allow me to encapsulate global test setup related to a particular class inside a category. I've missed this when using specta, so I implemented it in this pull request.

Here's the relevant Cedar info:

https://github.com/pivotal/cedar#global-beforeeach-and-aftereach