Add define_negated_matcher

This is PR is a result of the discussion and work done on: https://github.com/rspec/rspec-expectations/pull/505#issuecomment-40676532

TODOs:
- [ ] add cuke
- [ ] cleanup negation_spec
- [ ] add proper docs
- [ ] general cleanup

@myronmarston @JonRowe /cc
I think a better example might be `include` having a negative definition as `exclude` -- that's a case where the description isn't simply putting `not` in there.