add callbacks and fix new_recode? still true after saved.

I add callbacks before/after_create/save/update/destroy if it exists an instance method.
Not implemented specify by a class method.

I found a bug, new_recode? still true after saved.
use respond_to?

replace
respondsToSelector "after_fetch" ->
respond_to? :after_fetch
call super at ```awakeFromFetch``` and ```awakeFromInsert```
call after_destroy_callback if save was succeded
set @destroyed true if the save was success.
change order

set @destroyed true first.
because when called after_destroy_callback @destroyed should be true.
Merge pull request #8 from katsuyoshi/new_callback

add callbacks and fix new_recode? still true after saved.