Spy into double

Now double is spied without `spy_into`.
Fixes https://github.com/igrep/crispy/issues/4.
See details on updates of README.

Other changes:
- Make `SpyBase` to discard `SpyMixin`. I'm planning to abstract `Spy` and `ClassSpy` better with `SpyBase`.
- Discard `Stubber` and `WithStubber`. By attributing responsibility for stubbing feature to `SpyBase`, `SpyBase` reduces the number of modules the target class `prepend`-s. And it enables me to avoid method-redefinition-warnings more easily.