add jsdoc tags from jsdoc v3.2-dev

I've updated the syntax file to bring the JSDoc keywords up to date with the HEAD revision of jsdoc.

This file defines all the jsdoc tags: https://github.com/jsdoc3/jsdoc/blob/master/lib/jsdoc/tag/dictionary/definitions.js

Some keywords have been removed from jsdoc, but this pull request does NOT remove any existing keywords, in case you want to retain backwards compatibility.  I'm also submitting a separate pull request that does remove all old keywords.  Take your pick. :)

Here are the tags added:
 - abstract
 - callback
 - classdesc
 - defaultvalue
 - desc
 - emits
 - enum
 - external
 - fires
 - instance
 - listens
 - method
 - mixes
 - mixin
 - summary
 - this
 - todo
 - typedef
 - undocumented
 - var
 - variation
 - virtual