latex plugin name changed to render_math

The plugin name has been changed from latex to render_math as per advice from people on pelican IRC. In order not to confuse, the latex plugin is symbolically linked to the render_math plugin.

Besides the name change, the following has been added to the plugin:

 * Updated README
 * Extra options for template designers
 * Put mathjax script into a separate file

With regards to the latex folder, @smartass101 is going to be writing a LaTex reader. This plugin and his reader do two totally different, but there is a chance that people could get confused. I plan to add content to the latex folder explaining the difference so that a user can make an informed decision.