Update catalog page and add new "Who's Using VWF" page. 

First stab at splitting the catalog and adding a "Who's Using VWF" area. 

I did not follow all of Jess's recommendations for which demos to remove, so these are still up for negotiation. Namely,

1 - I left command center. While not overly interactive, it is an example of a scene with a lot of model data.
2 - I left Google Earth. I can go either way on this one... requires a plugin, which I don't like. 
3 - Moved the cesium app to the gallery, since AGI submitted the initial demo to us. 
4 - I removed chat, mainly b/c it's just unusable in it's current state. Can be added back in when its working well.
5 - I left sandtable, because we demo that one often. I can be persuaded to remove it though. 
6 - I left the two tile-puzzles b/c they're a good example of 2D vs 3D, and since it's the only 2D app we have fully working, I think it's important. We can eventually move this elsewhere, perhaps into a cookbook. 

Additional note: Gaining permissions from everyone to use logos on the "Who's Using VWF" is still in progress. I will hold off on merging until that is complete, but I wanted to start the conversation about which demos to leave and which to drop. 

@eric79 @jessmartin Please review, and provide feedback.