Pass example metadata to beforeEach/afterEach

Addresses issue #105, which essentially involves porting RSpec
example metadata to Quick. See: https://www.relishapp.com/rspec/rspec-core/docs/metadata/current-example

- Create ExampleMetadata class to encapsulate data on current
  executing example
- Add `beforeEach` and `afterEach` DSL that takes a closure with a metadata argument
- Add functional tests
I used Xcode autocomplete to expand these closures; didn't notice the ugly `() -> () in` here. I'll remove these in favor of a trailing closure.