New version of the customShapes plugin

This is an update of the existing customShapes plugin:

- New "Font Icon" mode (you can now display symbols / icons in nodes)
- New WebGL shape renderer (shapes, background images and font icons are supported)
- Small changes in the API + new docs

In order to achieve the best performances, the WebGL renderer uses a system of sprite caching for font icons and background images, while the border shapes are produced using real-time using polar drawing functions. 

This means there are some visual differences between the Canvas and WebGL version: *some things will look better in canvas, other better in WebGL* (see the new customShapes demos pages in the examples/ dir to try for yourself).

People that need icon fonts and WebGL shapes can use the plugin right now!

For those who want more control or better rendering, or more consistant look between rendering modes, I am still working on fixing these small issues (they take the longer time) here:
https://github.com/jbilcke/sigma.js-plugin-customShapes/issues