Mathjax2

OK, according to the review of the previous implementation [PR311](https://github.com/ralsina/nikola/pull/311), I am opening this new pull request and closing the old one.

The idea is the same as before... when you put **mathjax** as a tag from a post, the math in this post will be rendered by mathjax library, whereas if the post does not contain any math, mathjax will not be loaded. We suppor the math directive in rest and the math obtained with the ipynb plugin if you add the proper mathjax config file which is now customizable ;-)

OK, time to go to bed, see you tomorrow Roberto!

Damián.