Make remote group import configuration sensitive

In order to make the remote group import unobtrusive, the decision for
copying remote groups will be delegated to the harvester configuration.

The default case is, that remote groups will not be imported. If remote
groups should be imported which are present in local system the
configuration for "remote_groups" has to be set to "only_local".

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Add option to create non-existent remote groups

To complete this feature I have added another option for the remote
groups configuration field: create. In this case, the CkanCient is used
to retrieve the remote group details from the source CKAN.

Some fields have to be dropped, for instance a group's display name is
not intended to be set on creation time. These fields are dropped before
the dictionary containing the group details is passed to the CKAN action
logic.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>
Update README

I have added a description for the remote group import configuration to
the README file.

Signed-off-by: Konrad Reiche <konrad.reiche@gmail.com>