Add tern.js support

[gfwilliams]

tern.js knows all kinds of fancy stuff about JavaScript, and it even works with CodeMirror: http://ternjs.net/doc/demo.html

There's a minor issue at the moment to do with its (or acorn.js's) use of eval/new Function. It looks like this can be fixed with a sandbox though: acornjs/acorn#90

Also, tern uses a json file that describes the built-in JavaScript functions and what they return: http://ternjs.net/defs/ecma5.json

It should be trivial to automatically append Espruino's built-in functions with at least basic type information.

[gfwilliams]

done