Customer structure

[Trello Story](https://trello.com/c/9KRA0EpY)
- [x] Add Customer model and migration
  - [x] Add STI to Customer: Company and Person 
- [x] Add Address model, belongs to Customer
  - [x] Add State and City models, belongs to Address and populate them
  - [x] Add nested attributes on form
- [x] Add validations
- [x] Add seeds data
- [x] Add acceptance tests