add invokes(&block) to support fakes

The "fake" pattern (http://xunitpatterns.com/Fake%20Object.html) is an extension of stub where the method is actually implemented (by the tester) rather than simply returning a canned value.

The attached patch supports the following:

obj = stub()
obj.stubs(:foo).invokes do |bar|
  if bar == :one_thing
    :value_for_one_thing
  else
    :value_for_anything_else
  end
end

This allows you to handle multiple and differing calls to the same method in a single, clear, explicit declaration.