Refactor SharedExampleGroup to new Concerns module.

- Add #alias_shared_context and #shared_context aliases.
- Alias RespondTo#with_a_block to #and_a_block.

Current spec status:
- 2.0: 427 examples, 0 failures in 0.12451 seconds.
- 2.1: 492 examples, 0 failures in 0.11104 seconds.