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>

wincred/wincred_windows.go

@@ -33,7 +33,7 @@ func (h Wincred) Delete(serverURL string) error {
 func (h Wincred) Get(serverURL string) (string, string, error) {
 	g, _ := winc.GetGenericCredential(serverURL)
 	if g == nil {
-		return "", "", credentials.ErrCredentialsNotFound
+		return "", "", credentials.NewErrCredentialsNotFound()
 	}
 	return g.UserName, string(g.CredentialBlob), nil
 }