wms proxy

DO NOT MERGE or MERGE AT YOUR OWN RISK!

Down the rabbit hole, a suggested fix for #946. I saw that authenticated geoserver requests go via the proxy controller, so why not just run them all through?

Have a crack at:
http://dfruehauf.emii.org.au

Still more work to be done here. I think that in general there should be a designated `openlayers` proxy in addition to the ProxyController.
... and provide the implementation for `Ext.ux.Ajax.proxyRequest()`.

I haven't mentioned anywhere the main reason I think this is better is it isolates the construction/URL encoding stuff to one place instead of everywhere a URL is used.