Add in new methods which don't raise, and unify bang semantics across RbNaCl

Now all authenticatos, signatures and boxes have a non-raising version of their check/open function, which is the bang method, and a raising version which is the plain one.

This is a switch for authenticators and signatures, but it is consistent with the bang methods being dangerous (i.e. they could let an invalid signature pass unchecked and unnoticed - the side-effect is a possible lack of security.)