Added mathjax support through magic tags.

Welcome...
Here, we introduce a new concept: **"magic tags"**.
In this case, we have a magic tags called **%mathjax**.
If we want to post an article containing math text, we just have to set **%mathjax** as a **tag** and all the math text written in rst throught the *math directive* or using *ipynb plugin* will be rendered as a nice mathematical expression. Furthermore, all the post which don't have math text will **NOT** load the mathjax js, nice... don't you think?

Note: I also modified a little the index template to integrate tags, because I like to see the tags in the index and also because I need them to load mathjax if the index contain a post with math text. 

Gracias por el tweet, Roberto... estimulante... jeje.

Discussion and review welcome...