Add more PBJ support for Signatures

[lpetrovic05]

More support for java.security.Signature is needed by the Bytes class. Apart from adding data to a signature to be validated, the Bytes could also contain the signature itself, so we also need a methods for this. Convenience methods that might be needed in the future is to also add support for offset and length.

Tasks

• Add support for Signature.verify()
• Add overloaded methods with offset and length