NuCivic/nucivic-internal#153 Complete dkan_migrate_base to harvest data ...

...from a CKAN site.

NuCivic/nucivic-internal#153 

### Acceptance tests
* git pull
* drush en dkan_migrate_base
* drush mi --group=DKAN --uri=http://dkan -v
* Datasets, Resources (and files with allowed file extensions), Groups and Tags from http://demo.ckan.org should be present in the dkan site.