add is_logic about type_traits

[jzmaddock]

This is not what type_traits is for - we do not provide specialization points for other libraries, we introspect the type system, nothing more or less.

[gpeterhoff]

tribool is a component of boost. As I wrote, such libraries must of course set is_logic (to true) themselves. But first a basis has to be created to represent this.
As I wrote ages ago, I would like to provide classes for 4-value logic https://en.wikipedia.org/wiki/Many-valued_logic#Belnap_logic_(B4) and for 3-value logic (which could replace the old tribool).
These would of course contain an is_logic.
Now you have to decide:

• is_logic is not implemented
  you are stuck with the stone old code (tribool), without the possibility to extend it
• is_logic is implemented
  various logic classes can be categorized (there are even more than 2-, 3- and 4-value logics)

please open this issues.