Give objects without instance variables to users

[stcorbett]

Separate the original state of the records in an ActiveHash sub-class
from the records that are being passed around in the rest of the
application

[syguer]

@stcorbett
I agree to fix the problem.
This patch looks resolve the problem of surface, but not fundamental problem.
It seems that fundamental problem is the fact that ActiveHash instance is editable.
I'll try prohibit it on next version.

Could you wait it please?

[syguer]

Discussing on #146 so closing this.