Pass defer_commit in context on get_site_user calls

See ckan/ckan#1714. Until that is fixed properly, the `defer_commit`
flag avoids some `DetachedInstanceErrors` happening during the
harvesting.
Merge branch 'master' into 63-extend-spatial-harvesters

Conflicts:
	ckanext/spatial/harvesters/base.py
[#70] Fix deletion of harvested CSW records

The object id was pushed to the list returned by gather_stage before
being saved on the db, so None was added, causing an exception in the
Redis queue
Merge branch 'master' into 63-extend-spatial-harvesters
[#63] Add extra stuff to the get_package_dict extension point

Moved the call to get_site_user higher on base.py so it's available to
extensions. Also added the parsed XML etree so it does not need to be
parsed from the string again.
[#63] Added extension point for defining custom validators