Don't try and parse everything as a timestamp

This is bad, because dateutils.parse will successfully parse many things
which may not have been intended to be interpreted as timestamps
(e.g., '2'). Only try and parse it as a timestamp if we have reason to
believe it might contain one (i.e., its field name is 'timestamp').
Sanitise + stringify all sourcedict data to deal with lists
Merge branch 'fix-sanitize' of github.com:anchor/ceilometer-publisher-vaultaire into modified-impl-testable

Conflicts:
	ceilometer_publisher_vaultaire/vaultaire.py