Setting geometry type of new layers

[brandonreavis]

Right now to create a layer you need to pass the wkbGeometryType like:

var lyr = ds.layers.create('layername', srs, gdal.wkbPoint);

Since we have removed the need to use the wkbGeometryType enum everywhere else in the binding, it would be nice to remove it here too. We could pass the geometry constructor like:

var lyr = ds.layers.create('layername', srs, gdal.Point);