Added function for CPF validation

The Cadastro de Pessoas Físicas (CPF) – Portuguese for Natural Persons Register – is a number attributed by the Brazilian revenue agency (Receita Federal – Federal Revenue) to both Brazilians and resident aliens who pay taxes or take part, directly or indirectly, in activities that provide revenue for any of the dozens of different types of taxes existing in Brazil. By means of this universal number the Federal Revenue computers can estimate the income tax that is due, thus directing fiscalisation.
Merge pull request #705 from luksm/patch-2

Added function for CPF validation