Remote document loading isn't properly integrated via documentLoader API

[dlongley]

The remote document loading code (that uses a cache and some debouncing stuff) isn't properly integrated with the JSON-LD documentLoader API. This was noticed because Link headers weren't being parsed. I added a quick patch to call out to jsonld.documentLoader instead of using $.ajax, and then "injected" the @context from the Link header into the document. I left some notes about how this is less than optimal, but will at least get Link header support working for now.

We should try to figure out if we can have remote document loading go through the API instead of doing it externally -- or if this will make the debouncing/cache features too difficult to implement, etc. We should also add something to the UI to indicate a Link header @context was injected.