Add lj/webgl pair style

[andeplane]

We don't have cuda or anything like that, but we do have webgl. It is possible to do all computations in a simple pair style like LJ just to be able to run big simulations in the browser.

Maybe compute shaders are available on WebGL? If not, we can send all atom positions/types etc as textures into the fragment shader kernels and write forces and energies as output maybe. Problem is that we'd like one fragment shader kernel per atom i, not per Fx, Fy, Fz, E per atom. So maybe we can write to multiple textures, one for Fx, one for Fy etc.

https://gist.github.com/adrianseeley/f768fd7a3aab2370eafc
https://github.com/gnonio/gl-compute
http://www.khronos.org/webcl/
https://kripken.github.io/emscripten-site/docs/api_reference/module.html