[core] Add docs for the ES6 module system
[core] Various typos and line length fixes

In ES6 modules guide
[core] Add links to CommonJS and AMD module system docs
[core] Add link to API docs for Y.require()
Add ES6 entry to YUI core document
[core] Incorporate @tivac and @okuryu feedback
Merge branch 'es6-userguide' into live-docs
Merge branch 'live-docs' into dev-master