Display the current health of the datastore

[andylolz]

It would be useful if http://datastore.iatistandard.org/api/1/about (or another route) gave more details about the last crawler run e.g. when it happened and what happened. The ok=True and status='healthy' that are output at the moment are just hardcoded:

IATI-Datastore/iati_datastore/iatilib/frontend/api1.py

Lines 34 to 39 in 852ec9a

	return jsonify(
	ok=True,
	status='healthy',
	indexed_activities=count_activity,
	indexed_transactions=count_transaction,
	)

[markbrough]

This issue also makes it hard to diagnose the problem behind #268