Jinja2 2,7 compatibility
Rework query expressions in MOM.DBW.SAW

Implement MOM.DBW.SAW.QX to add support for:

* arbitrary query expressions

  first implementation of MOM.DBW.SAW failed for expressions like::

      (Q.races - 3) == Q.discards

* query expressions for composite attributes like::

      Q.manager.lifetime == ("1990/01/02", "2013/07/21")

* query expressions as arguments for `.attrs`

* multiple arguments to `order_by`

  + each argument can be an arbitrary query expression

* proper handling of aliases for attribute joins

Remove the module MOM.DBW.SAW.Q_Exp and various methods for dealing with
query expressions from MOM.DBW.SAW.Attr.