better autocomplete using tern.js

I had started implementing better autocomplete in https://github.com/Khan/live-editor/pull/178, but I didn't get very far with it. This morning I ran across a library called tern.js.

> Tern is a stand-alone, editor-independent JavaScript analyzer that can be used to improve the JavaScript integration of existing editors.

It supports the following things:

> - Autocompletion on variables and properties
> - Function argument hints
> - Querying the type of an expression
> - Finding the definition of something
> - Automatic refactoring

We probably don't care about automatic refactoring, but the others would be quite helpful. Although we do provide some argument hints, these are only for "built-in" functions. For more information see: http://ternjs.net/

Start by reviewing pull request #178 and the tern.js documentation linked in the issue. Compare the proposed approach with the requested autocomplete, argument hints, type queries, and definition lookup; the issue does not identify files, tests, or a narrower completion criterion.

Written by the indexing model from the issue text.