Changes to bools under Numexpr

[Ivan Vilata i Balaguer]

Hi all.  The attached diff makes some changes to Numexpr support for
booleans.  The changes and their rationale are below.

1. New ``True`` and ``False`` boolean constants.  This is so that 1 and
   0 are always proper integer constants.  It is also for completeness,
   but I don't envision any usage for them that couldn't be expressed
   without the constants.

2. The only comparisons supported with booleans are ``==`` and ``!=``,
   so that you can compare boolean variables.  Just as NumPy supports
   complex order comparisons and Numexpr doesn't, so goes for bools.
   Being relatively new, I think there is no need to keep
   integer-boolean compatibility in Numexpr.  What was the meaning of
   ``True > False`` or ``2 > True`` anyway?

3. This is also why casting booleans to normal numbers is not allowed,
   so ``prod()`` and ``sum()`` on booleans aren't either.  What is the
   meaning of boolean addition anyway?

To make it short, this patch is kind of a strengthening of boolean
values.  I expect some people to disagree with the changes, and that's
why I would really like to hear your opinions on it.

Regards,

::

	Ivan Vilata i Balaguer   >qo<   http://www.carabos.com/
	       Cárabos Coop. V.  V  V   Enjoy Data
	                          ""
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