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:

* Use 1.9 hash syntax in `delegate_method`
* Don't explicity test failure messages
Add more examples in README for DelegateMethod
Ensure independent matchers are widely available

The current form of including independent matchers for TestUnit does not
end up including it in `ActiveSupport::TestCase` tests in a Rails app.
This ensures that `Shoulda::Matchers::Independent` are available in
`ActiveSupport::TestCase`.