[1508] Bring in model and other bits from master.
[1508] Merged in a good chunk of ckan/ckanext-harvest master

* model is merged (albeit with the harvest_source_reference DGU modification still for now)
* NOT merged the change to harvest_source stored in SOLR as a package type
* enabling us to continue using the old Web UI (politics makes us) but it uses all the new model fields and logic functions
* NOT merged the move from carrot to pika/redis (to save time)
* ckanharvester is simply copied from master
* Tests fixed up

New functionality for DGU:
* Shows when a harvest is in progress

This is all tested manually with DGU's ckanext-spatial and seems fine.
Merge branch '1508-update-from-master' into 1508-update

Conflicts:
	ckanext/harvest/harvesters/base.py
	ckanext/harvest/harvesters/ckanharvester.py