lookuptable with example and docs

As we discussed [previously] (https://github.com/mrdoob/three.js/pull/4571) I've included a better example.
The model is a buffer geometry which represent an arteriovenous anastomosis between a radial artery and a cephalic vein. The 3D model was built from a CT image using [vmtk] (http://www.vmtk.org) and then I ran a 3D CFD simulation in order to have the pressure values on the surface.
The file size ( the one including buffer geometry attributes)  is only 1 Mb because I've decimated the original surface.
In this version of the lookup table I've also included the possibility to toggle a legend bar with different layouts, ticks and labels.
Build your lookup table from predefined color maps ( rainbow blue to red, cool to warm, black body radiation and grayscale ) and set the number of colours. 
Define the boundaries of your data (max and min values) and then obtain the corresponding color from the colormap.

Let me know if is ok or if I have to change/improve something.