Make hardware root-of-trust support more generic.

[cjpatton]

Attesation.ValidateSigner() in attesation.go verifies the identity of the issuer of an attestation. Currently the issuer is either a "key" (e.g. the public policy key) or a "tpm"; this check and verification of the signature are hard-coded into the routine. We could make this more flexible by overloading the auth.Prin type to implement signature verification:

type Verifier interface {
  Verify(text [] bytes) bool 
}

type Prin struct {
   Verifier
   Type string,
   Key Bytes, 
}