Change touch policy from always to cached

This will change the key that's generated when running `yubikey-agent -setup` to a key
with a touch policy of "cached". This will mean that "a touch is not needed if the YubiKey had been touched in the last 15 seconds, otherwise a touch is needed"

Fixes FiloSottile#146

setup.go

@@ -140,7 +140,7 @@ func runSetup(yk *piv.YubiKey) {
 	pub, err := yk.GenerateKey(key, piv.SlotAuthentication, piv.Key{
 		Algorithm:   piv.AlgorithmEC256,
 		PINPolicy:   piv.PINPolicyOnce,
-		TouchPolicy: piv.TouchPolicyAlways,
+		TouchPolicy: piv.TouchPolicyCached,
 	})
 	if err != nil {
 		log.Fatalln("Failed to generate key:", err)