[noticket] Add KML file datapreview to ckanext-spatial

Naive implementation of KML file preview using the GeoJSON preview and adding a Leaflet plugin to support KML. It's kept as a separate previewer in case the implementation needs to change to OpenLayers at a later stage.

![kml](https://f.cloud.github.com/assets/81432/922387/decb3aa2-ff26-11e2-9477-b839168c483c.png)