Be more careful about JSON date serializing

In [1]: from Products.CMFPlomino.PlominoUtils import json_loads, json_dumps, Now
In [2]: json_loads(json_dumps(Now()))
Out[2]: DateTime('2013/10/21 19:26:48 GMT+7')
In [3]: json_dumps(Now())
Out[3]: '{"datetime": "2013-10-21T19:26:58+07:00", "__datetime__": true}'
In [4]: Now().ISO()
Out[4]: '2013-10-21T19:29:25+07:00'
Test date serialization
DateTime can take a datetime instance, and pays attention to its timezone
Fix StringToDate invocation

'%z' in the format spec is invalid. This was raising a ValueError,
which was being caught. Pass format=None, so dateutil.parser.parse
is called. This understand the format we have here.
Test against the instance timezone
Merge pull request #452 from plomino/jean-json-loads-dumps

Be more careful about JSON date serializing