get rid of mx.datetime dependency

Hi, after some investigation, it seems that:
mx.DateTime has been created after a package called zxDateTime that has been integrated as the standard "datetime" module in Python 2.3 around 2003.
mx.DateTime might have a few extra, but overall I think we could get rid of that dependency.
t o
Why mx.DateTime sucks?
- it's one more dependency, something to package, to grab from some strange and bad looking site (image prejudice)
- it sucks to install for the poor developers stuck on Windows (I'm not one)
- it's backed by C only (no more compatible pure Python implementation), meaning that it suck especially to run OpenERP server on Jython or PyPy.

It seems that the mx.Datetime can be backed by the standard datetime package. Except may be some parsing features (TODO ensure). In Tryton for instance, they got rid of mx.DateTime entirely already.

mx.RelativeDateTime might be backed by datetime.timedelta.
or may be: from dateutil.relativedelta import *

I discussed that with Jython/Python specialists, like Jim Baker and all agree on that.

Also, if something is missing, it looks like the most standard way of getting those extra miles should be using the following library instead:
http://labix.org/python-dateutil
see http://mail.python.org/pipermail/tutor/2005-October/042459.html for example

Hope we could do that house cleaning.