fabric: Added "update_mapproxy" task
fabric: Added "clean_mapproxy_cache" task
mapproxy: Added "airspace+airports" layer

This essentially renames the old "airspace" layer to "airspace+airports" and
creates a new "airspace" layer with just the airspace. This can be used by the
proSoar project to not have to run a separate tileserver for this anymore.
map.js: Use new "airspace+airports" layer
Merge pull request #254 from Turbo87/mapserver

Extracted pure "airspace" layer in MapServer config
mapproxy.yaml: let mapproxy combine the layers, not the mapserver