HTML syntax checker

This adds syntax checking (not validating).
The parser is taken from https://github.com/aredridel/html5/ and is modified (updated version will be available later there).  This was packed by browserify.

Keep in mind: this parser implements true [**html** parsing algorithm](http://www.whatwg.org/specs/web-apps/current-work/multipage/parsing.html#parsing) (instead of non-strict xml). Some cases can be unexpected.
I'm  also planning to implement html5 *validation*.

It seems we need to disable checker in inheriting modes.
I think doctype reports must be replaced by message like "Doctype not found. Assuming fragment mode."

https://github.com/ajaxorg/ace/issues/1181