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Some servers provide an etag which is a unique identifier for the
content being served (not always a hash).  This adds two new column to
the archival, an etag and a last-modified field.  If the etag is
available in the archival, and we get the same etag from the request,
then we will not download the file.  The last-modified field is saved
for sanity, it isn't yet used with If-Modified-Since in the request.

This requires a migration:

    alter table archival add column etag character varying;
    alter table archival add column last-modified character varying;

Notification of package updates is only done if at least 1 resource was
downloaded, to avoid unnecessarily running QA and Packagezip.

Fix slow import, delay loading celery

Fix tests

Copy CKAN extension boiler-plate from core ckan 

Conflicts:
	setup.py

Copy CKAN extension boiler-plate from core ckan template: paster --plugin=ckan create -t ckanext ckanext-archiver

Small changes

To remove deprication warnings

Archive packages2 WIP

Compatibility with requests==2.7.0 - response.headers is a funny dict which doesnt JSON dump unless you cast it.

Test IPipe interface is called when a resource is archived

Fix for difficulty archiving "private" datasets in an group/organization. Still need to solve https://github.com/ckan/ckan/issues/1975 for groups.

[noticket] Fix for saving filenames with unicode chars in. e.g. audit-commission-expenses-october-2013/5b1d00ae-2d6c-4016-8879-50b4ba19a114.