crypto/openpgp: support public keys generated by End-to-End

packet/packet.go: add ECDSA to the algorithms in CanSign()
packet/signature.go: add support for the features subpacket (which is
used to signal support for the MDC packet)
openpgp/read_test.go: add a key generated by End-to-End as a testcase
(End-to-End's keys are unusual: they set critical bits)

Change-Id: Ifaf3922cd35ea27cf1276d7a8238dd50536eb4ed
Reviewed-on: https://go-review.googlesource.com/2058
Reviewed-by: Adam Langley <agl@golang.org>

openpgp/packet/packet.go

@@ -427,7 +427,7 @@ func (pka PublicKeyAlgorithm) CanEncrypt() bool {
 // sign a message.
 func (pka PublicKeyAlgorithm) CanSign() bool {
 	switch pka {
-	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly, PubKeyAlgoDSA:
+	case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly, PubKeyAlgoDSA, PubKeyAlgoECDSA:
 		return true
 	}
 	return false