Be more careful about JSON date serializing

This makes `json_*` more robust in Plomino context. If you want to generate JSON for something external, you may need to stringify dates before serializing.

```python
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'
```