Skip List for GLIB

[Eric Vander Weele <ericvw gmail com>]

Hello,

Before I started working on this, I wanted to bounce the idea of adding a skip list -- GSkipList.  It shouldn't be that more complicated than the balanced binary tree implementation and the test driver for a skip list would almost be the same as the the one for the balanced binary tree.

If this sounds like a good idea, I can implement this either as a set or a multi-set.  Either implementation is fairly straightforward and I can implement both until one is decided upon.

The README suggested that I ask first, especially since I am new to this.  

Thanks,

-eric