[#56] Add EC API Changelog mock request

Conflicts:
	ckanext/glasgow/tests/mock_ec.py
[#56] Add changelog_show action for requesting changes to the EC API

Right now it only forwards the request to the EC API with the same
parameters, but we might want to tweak the results at some point.

Only sysadmins can use it.

Conflicts:
	ckanext/glasgow/logic/action.py
	ckanext/glasgow/plugins.py
	ckanext/glasgow/tests/logic/test_action.py
[#56] Changelog API returns a list of Audits
Merge branch 'master' into 56-changelog-update
Tweak validator to avoid indexing errors
[#56] Add more API endpoints
[#56] Abstract base harvester class to hold common methods
[#56] Firt pass at the changelog harvester

Currently handles dataset creation and updates.

TODO:
* persist the last processed AuditId
* Make sure we only apply the most recent update for a given object
[#56] Use Service to Service auth on Changelog EC API requests
[#56] Store last harvested AuditId

The way the changelog API works, you need to provide a starting AuditId
from which the more recent ones will be returned. We need to persist
this somehow, so we've added a DB table to store it with a timestamp.

The provided AuditId is returned as well, so we need to skip the first
item.

DB tables are created on startup if not already there.
[#56] Only process the most recent update audits

As there might be multiple update audits against the same object and we
have now way (nor interest) of getting each particular version, we only
process the most recent one, which will get the latest version of the
object anyway.
Merge branch '56-changelog-update'