Basic verification of class method w/o using spy_on

I noticed that basic give/verify functionality of class methods wasn't working they way I expected. The behavior was stubbed out, but verification was failing. For example,

``` ruby
class Dog
end

give!(Dog).woof { puts 'BARK!' }
Dog.woof # outputs BARK!
verify(Dog).woof

```

Would result in an error that woof had never been called. I added an invocation increment to the behavior injected by GivesClassMethods. I also added a spec. I'm not sure if this is actually something you want to support, but it seems useful and no other tests broke. Thanks!