Don't require PostGIS unless using it as a search backend

[amercader]

There is absolutely no reason to require PostGIS if you are using one of the Solr backends. It's just required for historical reasons as it was the only backend supported initially.

The PostGIS search backend is not recommended anyway and the PostGIS install adds complexity and causes issues like ckan/ckan#3929 or #182.

[amercader]

A bit of refactoring on plugin.py to not initialize the tables or store the extent if not using the postgis backend should be enough. And ideally add some tests using the solr backend.