if/unless option for callbacks about couch_potato

[kazjote]

I reimplemented callbacks in http://github.com/kazjote/couch_potato/commit/7ae9a51e0d836992df356fb259b3ae1f3824a09e using activemodel which provides if/unless functionality.

Whole testcase is unmodified (except for
spec/callbacks_spec.rb) and passes.

The code is much shorter and and will be even more if I modify specs to reflect new code base.

I think that there is no point in not using activemodel as dependency. Community put much effort in extracting activemodel and now it is really simple and awesome.

If you agree on accepting my changes I will change the specs too and make pull request.

[langalex]

great work. i'll pull this next week, when i have returned from traveling.