Slice cone exercise

This visual exercise show the principles of conic sections outlined in the video:

Introduction to Conic Sections - 
http://www.khanacademy.org/video/introduction-to-conic-sections

It could be an addition or replacement of the existing exercise:

http://www.khanacademy.org/exercise/recognizing_conic_sections

The only dependencies are Raphael and jQuery. 

This is a first commit to see how you guys like it, what needs to be added and or changed.

Sandcastle link: http://sandcastle.khanacademy.org/media/castles/Lovinglobo:master/exercises/slicing_cone.html