[model] association between country and states and refactor method to find states by iso
[migration] change numcode with iso code
[spec/models] refactor to use iso instead of numcode
[seed] change states seed to use iso instead of numcode
[migration] remove unnecessery migration for states
[specs/model] add few specs for attr_accessible attributes
Merge pull request #174 from Genshin/refactor_state_country_assoc

Refactor state country assoc