Add pagination support to harvester

Remote files will paginated (via adding ?page=x) until:

* The same content is returned (no pagination supported)
* A 404 is retunrned
* An empty document (ie not datasets) are returned

Pages start at 1, so eg /catalog.json is the same as
/catalog.json?page=1
Add first version of CKAN to DCAT converter
Add some basic tests and examples
Add enpoint for DCAT JSON interface

Uses the search to retrieve the CKAN datasets fast and transformt them
to DCAT. Supports page and modified since params. Number of datasets per
page and endpoint route can be configured via
ckanext.dcat.datasets_per_page and ckanext.dcat.json_endpoint
respectively.
Ignore auth when creating datasets from the harvester
Only return actual datasets
Allow custom harvesters to modify the package dict