core: add itemtypes datetime and timestamp

[Morg42]

Adds item types datetime (datetime.datetime object) and timestamp (POSIX timestamp implemented as float).

datetime items have additional methods item.as_ts() and item.as_str(format), which output timestamp or text respectively. format parameter is optional (as for strftime), None means ISO format.

timestamp items have additional methods item.as_dt() and item.as_str(format), which output datetime or text respectively. format parameter is optional (as for strftime), None means ISO format.

As of now, both types can not be edited in AdminUI as they are not yet implemented there; both will be displayed correctly. I modified the Admin-Module as to enable transferring datetime as JSON (json.encode added the default=str parameter). This might have to be revisited later, as of now, I see no problems.

Feel free to try out and find errors ;) most probably in timezone conversion, though I tried to work with tz-aware datetimes everywhere and get the timezone from shtime.tzinfo().

Oh, and I used datetime and timestamp as item type codes; but I'm thinking of changing those to dt and ts respectively. Any opinions on this?

closes #421