MDL-43856 MathJax: Add a mathjax loader filter.
MDL-43856 MathJax: alter the equation editor to make it use MathJax if it is available.

If MathJax is loaded by any means, the editor should use it instead of the filter. This prevents
slow ajax requests and gives much better rendering. MathJax could be loaded by the filter,
or it could just be done the old way by sticking the include in the page header.
MDL-43856 MathJax: Add to standard plugins list
MDL-43856 MathJax: Improvements to the MathJax filter

* Remove all traces of mathjax from the atto equation editor - it is filter agnostic
* Add a M.core.event module for core events that can be published/subscribed to by plugins (loose coupling)
* Add a FILTER_CONTENT_UPDATED event so plugins can tell the filter system when nodes are added to the page
* Implement a listener for this event in the MathJax filter
* Fire this event from the equation editor and from the glossary filter
* Add a nolink tag to the mathjax filter, so equations will not be processed by the other filters
* Add a smarter throttle function to the equation editor preview updates (Y.throttle is dumb)
MDL-43856 MathJax: Coding style cleanups.