revised the latex plugin so that it will work under both http and https protocols

The previous implementation of `latex` plugin can only work under http protocol, because it uses the http version of `MathJax`'s CDN, which will be blocked by some browsers by default if the website is under https. This will cause the equations fail to be rendered.

This commit utilizes `MathJax`'s [dynamic loading](http://docs.mathjax.org/en/latest/dynamic.html) with [https URL of CDN](http://www.mathjax.org/resources/faqs/#problem-https) to make the plugin work under both http and https protocols, with the capability to use the correct URL accordingly.

It has been tested in IE 9, IE 10, and Chrome 30.0.1599.69. An example under https can be found [here](https://grapeot.me/some-technical-details-about-kinectfusion.html).