[Feature] RG validation (just General Registry for now)

[raphaklaus]

Implement the validation for RG's (just General Registry for now, not including UF specific RG's) class, and tests

[the-darc]

I don't know if there is a way to validate RG.

See: http://pt.stackoverflow.com/questions/71/valida%C3%A7%C3%A3o-de-rg