This adds finally support to sign and verify messages using an RSA
private and public key.
The method used is PKCS1v15, as it was the easiest to implement first.
There is also PSS available in go, so that could be implemented later.
Go vet reported a problem with the interface of WriteTo returning an
int.
Line 13: method WriteTo(stream io.Writer) (int, error)
should have signature WriteTo(io.Writer) (int64, error)
To fix that problem, the interface was changed to return io.WriterTo,
which uses int64 for return values.