Avoid hitting length limit for GET requests

[springmeyer]

When serving WMS for stylesheets with many layers, and layers with long names, its easy to hit the limit for length of GET requests for a given browser.

A "meta layers" hack was added to work around this problem for a haiti deployment of the ogcserver: https://github.com/mapnik/OGCServer/blob/master/ogcserver/common.py#L384 and this needs to be removed and proper support added.

[manelclos]

The natural thing to do would be to follow the WMS spec, and allow Layers to contain other Layers (wms 1.1.1 spec: 7.1.4.5.2). But the support must be added to mapnik first.

[manelclos]

Initial support can be added with:

[map]
wms_groups = streets, buildings

[group_streets]
wms_title = Streets
wms_layers = streets_layer1, streets_layer1_labels
wms_name = streets