Class spy

Added a feature of class spy: log methods called by any instance of a class.
Partially fixes https://github.com/igrep/crispy/issues/1 (I've no time to test ClassSpy's stub!)

Other changes:

- No more support for rubinius. I'm not interested anymore because I don't use.
- make spies stoppable.
    - This is useful when you avoid to log unexpectedly. I need this feature because ClassSpy unexpectedly logs its receivers' `==` method and `inspect` method in test code.