Add SHA512 HashGeneratorFcn

Closes #1.

scram.go

@@ -9,6 +9,7 @@ package scram
 import (
 	"crypto/sha1"
 	"crypto/sha256"
+	"crypto/sha512"
 	"fmt"
 	"hash"
 
@@ -29,6 +30,10 @@ var SHA1 HashGeneratorFcn = func() hash.Hash { return sha1.New() }
 // to create Client objects configured for SHA-256 hashing.
 var SHA256 HashGeneratorFcn = func() hash.Hash { return sha256.New() }
 
+// SHA512 is a function that returns a crypto/sha512 hasher and should be used
+// to create Client objects configured for SHA-512 hashing.
+var SHA512 HashGeneratorFcn = func() hash.Hash { return sha512.New() }
+
 // NewClient constructs a SCRAM client component based on a given hash.Hash
 // factory receiver.  This constructor will normalize the username, password
 // and authzID via the SASLprep algorithm, as recommended by RFC-5802.  If