Add client functions to allow integrations within other CLIs.

This is a simplification of how the docker engine implements
this feature, but it will be ported there once this is merged.

Signed-off-by: David Calavera <david.calavera@gmail.com>

credentials/credentials.go

@@ -10,6 +10,13 @@ import (
 	"strings"
 )
 
+// Credentials holds the information shared between docker and the credentials store.
+type Credentials struct {
+	ServerURL string
+	Username  string
+	Secret    string
+}
+
 // Serve initializes the credentials helper and parses the action argument.
 // This function is designed to be called from a command line interface.
 // It uses os.Args[1] as the key for the action.