[#8] Use site user for harvesting actions

You don't need to create a 'harvest' sysadmin user any more.
By default this will be the internal site admin user. This is the
recommended setting, but if necessary it can be overridden by
the `ckanext.spatial.harvest.user_name` config option, eg to
support the old hardcoded 'harvest' user
[#8] Minor tweak to source_config
[#8] Provide a config option to continue after validation errors

This can be set instance wide on the ini file with

ckanext.spatial.harvest.continue_on_validation_errors

or per source, adding continue_on_validation_errors=true to the source
config.
[#8] Add get_extra to SpatialHarvester class
[#8] Use plugins toolkit whenever possible
[#8] Add support for import command to new harvester

The import CLI reruns the import stage for the last current objects, so
when running it, the previous objects don't need to be changed. Any
date check is overridden to force the update of the package.
[#8] Clean up base harvester, docstrings, some pep8