Complete removal of MathJax library.

This commit is intended for projects which don't need the Mathjax functionality. Fixes #549 
NOTE: Run `bower prune` and `gulp` to cleanup your working directory.

Mathjax is extremely time-consuming to build and very big: Removing it, removes 174MB footprint in Stackedit
and lowers build times from 33s to 25s (a 25% performance improvement running `gulp`)

**NOTE** I would prefer this to be a **separate branch** in Stackedit... either a branch where we explicitly include Mathjax or explicitly exclude it. @benweet I need your guidance here!