[ADD] JSONify Date objects to a UTC datetime in OpenERP server format

This way, the client can send genuine local Date objects without
caring too much, the server will get them in an expected format. This
can then be used by leaving Date objects in domains, and have these
date objects end up correctly in the server without creators of
domains having to manually serialize them to UTC from a local datetime
somehow.
[ADD] ways to correctly handle local datetimes in e.g. domains

* change datetime.now() to generate user-local naive datetimes
* add datetime.utcnow() behaving as the old now()
* add date.today() generating a user-local date (for the current day)
* add datetime.replace() to replace any specific attribute of the
  datetime object (except tzinfo, for now)
* datetime.toJSON() now returns the equivalent javascript Date object
  (warning: uses the datetime attributes directly, since datetimes are
  naive if they were created with utcnow() the Date result is going to
  be complete nonsense). With the previous commit datetime.now()
  generates a user-local now() which is converted to the correct UTC
  datetime when sent to the server.

This means it becomes possible to generate datetime bounds for the
user's local today with either

    datetime.datetime.now().replace(hour=0, minute=0, second=0)

or

    datetime.datetime.combine(
        datetime.date.today(),
        datetime.time())

and once send over JSON-RPC the server will get the local datetime
in UTC to the server's format.

nb: user-local means "in the timezone of the user's browser" in this
context.
[MERGE] [ADD] ways to correctly handle local datetimes in e.g. domains

* Alter datetime.now(), generate a local datetime (add utcnow() which generates a UTC datetime)
* Implement datetime.replace() to manipulate local datetimes
* Implement date.today(), generates a local date
* Implement datetime.toJSON(), returns a javascript Date (assumes datetime attributes are local)
* Add conversion hook in JSON and JSONP handlers, automatically converts a Date object to a UTC datetime formatted according to server formats

Should allow the generation of correctly working (from the end-user's POV) [Today] filters, amongst other things.
Eg: a local expression in a domain for 'Today 00:00:00' would now be expressed as 'datetime.datetime.now().replace(hour=0, minute=0, second=0)' (no .strftime) and will be converted to UTC when sent to the server