"veyron/security": Expose Load/SavePEMKey

LoadPEMKey and SavePEMKey methods would be needed by WSPR to manage
its private key within Chrome. This CL exposes these two methods.

Change-Id: I8e16fccce899ed305471549978a33d24c2453130
diff --git a/security/principal_test.go b/security/principal_test.go
index fd6d05d..8be6f25 100644
--- a/security/principal_test.go
+++ b/security/principal_test.go
@@ -103,7 +103,7 @@
 		panic(err)
 	}
 	defer f.Close()
-	if err = savePEMKey(f, key, passphrase); err != nil {
+	if err = SavePEMKey(f, key, passphrase); err != nil {
 		panic(err)
 	}
 	return dir