MathJax support added

I see that there was a brief discussion of adding MathJax support in the past, but I could not find a pull request. As a result, I have implemented MathJax support with the current version of sublimetext-markdown-preview. I have tested this change on OSX and Windows 7.

## Usage

MathJax support can be turned on or off in the MarkdownPreview.sublime-settings. When on, math can be rendered in the browser (with a connection to the internet) with the following:

- Inline math `\\(\frac{\pi}{2}\\)`.
- Display math `$$\frac{\pi}{2}$$` or `\\[\frac{\pi}{2}\\]`