Add a keyring based Storage.

Reviewed in http://codereview.appspot.com/6450070/.

Fixes issue #21.
diff --git a/samples/keyring/README b/samples/keyring/README
new file mode 100644
index 0000000..26e81d8
--- /dev/null
+++ b/samples/keyring/README
@@ -0,0 +1,4 @@
+Demonstrates storing Credentials in the system keyring.
+
+api: plus
+keywords: cmdline pagination oauth2