Leaflet Dynamic Menu

This pull request includes a leaflet application that has a more dynamic layer menu. Previously we provided a static list of layers. This pull request includes a leaflet application called 'explore' that offers the user a dialog box with two lists, one is a dynamic list of TileStream layers available, and the other is a dynamic list of GeoServer layers available. 
 
Features:
Add layers from Tilestream server to the map
Add layers from GeoServer to the map
Remove Layers from the map
Switch on and off any added layers

Know short-comings: 
This application has not been added to the OQP menu tree - it still needs to be decided where it's home will be, or if it's features will simply become part of all of the leaflet apps.
The layers that are exposed via the layer list have obscure and ugly names, clean up is needed.
The leaflet layer control (when empty) is a bit fidgety. 
Clearly more features could be added to this app to make it more effective like a layer search tool.

https://bugs.launchpad.net/oq-platform/+bug/1197777