DSL: pass example metadata to beforeEach

Begins work on 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 DSL that takes a closure with a metadata argument
- Add functional test as a proof-of-concept
DSL: add example metadata to afterEach

Continues work on issue #105, passing example metadata to afterEach
closures as well.
Add functional tests for example metadata

Add functional tests that confirm example metadata is passed to
beforeEach/afterEach correctly. Completes #105.
Merge pull request #120 from modocache/105-example-metadata

Pass example metadata to beforeEach/afterEach