added /last wsapi method

@shane-tomlinson:

Implements /wsapi/interaction_data/last using limit:1 and descending:true, instead of grabbing all of the results and looking at the last one (https://github.com/mozilla/kpiggybank/pull/18)

assumes uuids are not generated randomly, as set in default.ini or local.ini:
[uuids]
algorithm = sequential