Signature.go should match Verify implementation and use crypto.PrivateKey

[melzhan]

Signature.go Line 30:
To match the Verify() implementation, this should be a crypto.PrivateKey and then a type switch should be used to provide algorithm specific code paths.

Otherwise, if the current function signature gets traction, introducing an algorithm agnostic counterpart will end up being a bit cumbersome.

[gmacf]

Fixed in #79