Make documentation example consistent with other examples.

Elsewhere we are using `# =>` to display the expected output and not
using `Test::Unit::Assertions`.
Make whitespace consistent.
Add `Mock#responds_like_instance_of`. Fixes #119.

This is useful if you have a class that is awkward to instantiate. The
implementation uses `Class#allocate` to instantiate the responder
_without_ calling the class' `#initialize` method. Having built the
responder instance, the implementation is the same as for
`Mock#responds_lie`.

Thanks to @simao for the original suggestion.