adding jsonld.js

This adds [jsonld.js](/digitalbazaar/jsonld.js), a Javascript implementation of the [W3C JSON-LD spec](http://www.w3.org/TR/json-ld/).

The version included was pulled from [https://github.com/digitalbazaar/jsonld.js/releases/tag/0.2.8](0.2.8), while I was assuming the npm integration stuff would handle minimization.

Hopefully the npm stuff is right: is there a right way to test the extraction? Will it backfill the older versions?