Restore delegation matchers
Restore delegate_method matcher

Matcher was removed in commit 77dc477c due to issues described in
https://github.com/thoughtbot/shoulda-matchers/issues/252

We've moved away from using Bourne and Mocha's `HaveReceived` matcher and
implemented a `StubbedTarget` class for spying on methods to be delegated.
We also removed the stub call to be handled via `define_method` for
framework-independent support.

Other changes include:

* Moving from `should` syntax to `expect`
* Use 1.9 hash syntax in `delegate_method`
Standardize failure_message method name
Refactor based on PR feedback
Properly stub target on instances
A few more updates from PR feedback
Add some more examples to README
Clear up method_on_target and PR comments

Don't have a method named method_on_target which behaves unexpetedly.
Instead use a method `#stubbed_method` to handle `as` transition.
Add examples for Test::Unit
Don't explicity test failure messages