Define API for Carto layers order

//         BOTTOM v     v TOP
client.addLayers([A, B, C]);

// 0 < 1 < 2

A.setOrder(0); // A, B, C
A.setOrder(1); // B, A, C
A.setOrder(2); // B, C, A
A.setOrder(-1); // throw error: out of range
A.setOrder(3); // throw error: out of range

A.bringToFront(); // same as A.setOrder(<layers length>);
A.brintToBack(); // same as A.setOrder(0);

cc @csobier @alonsogarciapablo @IagoLast

[IagoLast]

LGTM!

[Jesus89]

Closed by: https://github.com/CartoDB/cartodb.js/pull/2031