Adding start of lattice boltzmann

A git submodule is added that refers to the original code. The idea would be to
incorporate in the text directly via [dexy](http://dexy.it/), to show actual
testing rather than toy problem stuff. We can always remove it later on.
removing submodule
Revert "removing submodule"

This reverts commit 46ee45607eb3d363f761aab6c5f40abdc45a3877.
Submodules are difficult to deal with. Point well made.

Removing submodule the right way around.
Adding index.md file for easy serve of LB stuff

The LB stuff should be available as http://{base}/lattice-boltzmann, where
{base} could be the github page location, the localhost, or something ...
Merge pull request #5 from UCL/gh-pages

Initial description of a Lattice-Boltzmann chapter