Ability to pass additional options to L.StamenTileLayer

The `L.TileLayer` constructor takes an `options` object as a second argument, which `L.StamenTileLayer` ignores. I've patched it so that:

* the user can send an `options` object to StamenTileLayer's superclass;
* options like `minZoom` and `maxZoom` are still overwritten by StamenTileLayer to ensure we don't get outside acceptable bounds.

There are at least two useful options one can now use:
* `detectRetina: true` makes Stamen Maps look nice and crisp on high-DPI displays;
* `opacity: [0-1]` to control layer opacity.

Et cetera.