Create a new Budget::Phase model that:
* Stablishes a relation with its budget
* Stablishes relation with two other Budget::Phases (previous and next)
* Validates basic dates range, kind and description rules.
* Adds scopes to get the ones enabled as well as each individual phase
Create a factory that generates a basic and valid Budget::Phase
Create a model spec that checks kind, date range and budget validations.
