crypto/rsa: support unpadded signatures.

crypto/rsa: support unpadded signatures.

Usually when a message is signed it's first hashed because RSA has low
limits on the size of messages that it can sign. However, some
protocols sign short messages directly. This isn't a great idea because
the messages that can be signed suddenly depend on the size of the RSA
key, but several people on golang-nuts have requested support for
this and it's very easy to do.

R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/44400043