Coerce provsys support notes to strings for aggregation

I discovered to my horror that provsys support notes aren't guaranteed
to be strings - berth140 has some datestamps in there, and provsys is
returning them as python datetimes... wtf! I guess it does some "smart"
detection and types the data accordingly.
---
>>> r = Resource.get(8205)

>>> r.name
'berth140'

>>> r.details['notes']
datetime.datetime(2012, 6, 20, 0, 0, tzinfo=<provisioningclient.lib.dateutils.FixedOffsetTimezone object at 0x1bb8610>)

>>> print r.details['notes']
2012-06-20 00:00:00+00:00