Ol3 viewer

Adding this OpenLayers preview allows lots of geo-data to be previewed in CKAN, which seems like a great positive. I've not looked too much at the detail of this - perhaps @amercader could review?

The only major consideration is the emergence of the Resource Views feature - integration with that would be needed in the next few months, when CKAN 2.3 comes out.

We'd also need to note in the README about the code files that are copied from openlayers, and that those files have a BSD licence.