Adding start of docs giving all equations

Doc can be compiled with [dexy](http://dexy.it/). It should have a Julia filter
to weave julia stuff in.

~~~~~~~~~~{bash}
> pip install dexy
> cd LatBo/doc
> dexy setup
> dexy
~~~~~~~~~~

To recompile the docs, only the last line is needed.
Output is in doc/output directory.

Goes partway to #3.
Merge branch 'master' of github.com:UCL/LatBo.jl

* 'master' of github.com:UCL/LatBo.jl:
  Adding Travis build status