[forms] propagate aguments given to .vocabulary to .choices if it's a callable.
So we can properly uses vocabulary(limit=xxx) in places where we used to call
.choices(limit), since field.choices is not meant to be accessed directly.
Also, document expected behaviour for custom choices method for relation fields.
This directory contains extra libraries which are needed
to make cubicweb work.
The mx.DateTime python implementation is directly taken from
the mx.DateTime distribution. The only modification is the
strptime function which has been mocked using the standard
``datetime`` module. (as provided by the python2.5's stdlib)