Restore delegation matchers

This reverts [77dc477](https://github.com/thoughtbot/shoulda-matchers/commit/77dc477ce470e3b5a7d4f5f7a64972007643bb43) restoring the delegate_method matcher removed in should-matchers 2.0.

Some minor updates were made to get the delegate_method matcher working again however most of the original code is in tact.