Implemented validation for CPF with repeating digits.

Signed-off-by: Adriano Ueda <adriano.ueda@gmail.com>
Implemented validations for CPF numbers with same digit. This change fixes #2.

CPF numbers with same digit will not be checked as valid, despite of being correct by the CPF algorithm.

This change includes these numbers:

000.000.000-00
111.111.111-11
222.222.222-22
333.333.333-33
444.444.444-44
555.555.555-55
666.666.666-66
777.777.777-77
888.888.888-88
999.999.999-99

Note: only the 000.000.000-00 and 999.999.999-99 numbers are marked as invalid by Receita Federal. See http://www.receita.fazenda.gov.br/aplicacoes/atcta/cpf/consultapublica.asp