Add VerifiedDouble.any_instance_of(klass).

* Warning: only work with should_receive. Doesn't work with expect/allow
  syntax.
Raise error if any_instance_of is used if expect/allow syntax.
Implement describe "contract" feature
Add VerifiedDouble.any_instance_of(klass).

* Warning: only work with should_receive. Doesn't work with expect/allow
  syntax.
Raise error if any_instance_of is used if expect/allow syntax.
Merge: describe "contract" syntax. [#27]