Items with timedelta values are not export/importable as JSON

[toirl]

Timedelatas are not JSON serialisable on default and Ringo does not support to serialise these values yet.

So If you try to export items having a timedelta values using the ringo-admin command:

ringo-admin db savedata items

it will result in

File "/home/torsten/Entwicklung/ringo-apps/trainable/lib/ringo/ringo/lib/imexport.py", line 37, in default
    return json.JSONEncoder.default(self, obj)
  File "/usr/lib/python2.7/json/encoder.py", line 184, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: datetime.timedelta(0, 818) is not JSON serializable