Allow AND NOT in peering expression

This patch should address the issue raised by Aris in NCC#2014050637

In example 6 from https://tools.ietf.org/html/rfc2622#section-5.6 the "AND NOT" operation is used as an example. Semantically it does exactly the same as EXCEPT, but in the wild we've noticed that people expect to be able to use AND NOT instead of EXCEPT. 

It might be good to solicit more community feedback to establish whether we have a shared understanding of this principle.