Add MathJax to the blog.

The the javascript to use MathJax to render latex equations was added to
_includes/head.html file was. A post was also created to explain how
this was done.
Improve the MathJax configuration (add my macros)

Added a javascript file 'public/javascripts/mymathjaxdefs.js' defining
macros I use to type vectors and matrices in equations. The MathJax
configuration in head.html was updated to load this file alongside
Mathjax.

All of this configuration is described in the updated post
"using_latex_equations_in_jekyll".
Publish the post about MathJax configuration

The post was moved from _drafts to _posts and renamed to have a proper
date.