[v3] C++0x type_traits post-oxford additions

[Benjamin Kosnik]

> make_signed/make_unsigned require:
>> Requires: T is a cv integral type or enumeration, but not a bool type.
> 
> I.e. they are not supposed to work with floating point types.

Ack! Sorry, I thinking all arithmetictypes were valid. Maybe I messed up 
and was looking at the wrong draft of this, again.

I'll go ahead and simplify.

Thanks!

-benjamin