Pattern Matcher: Implement GreaterThanLink

This block of code implements the GreaterThanLink, as a special cse of a VirtualLink.

Here, VirtualLinks are links that behave as if they are in the atom space, but aren't actually there (because there would be too many of them).  When the pattern matcher trips over one, it evaluates it, and then behaves as if it found it in the atomspace (or not).