move auth out to its own file

auth.go

@@ -0,0 +1,14 @@
+package main
+
+import (
+	"crypto/rsa"
+)
+
+type Auth struct {
+	Nick string
+	Key  *rsa.PublicKey
+}
+
+func (a *Auth) Kind() string {
+	return "auth"
+}

client.go

@@ -18,15 +18,6 @@ import (
 	"unicode"
 )
 
-type Auth struct {
-	Nick string
-	Key  *rsa.PublicKey
-}
-
-func (a *Auth) Kind() string {
-	return "auth"
-}
-
 type ReadWriter struct {
 	io.Reader
 	io.Writer