Do not talk about HTTP clients but Context Loaders in API, Karlton was right again

It may be that other protocols are used to download contexts. See Dave's comment at https://github.com/json-ld/json-ld.org/issues/213#issuecomment-13623166.

I renamed the callbacks to LoadContextCallback and ContextLoadedCallback. The option in JsonLdOptions is now simply called loadContext making it a nicer interface (options.loadContext(url)).

I will close #213 directly. @dlongley, feel free to reopen the issue if you feel the naming is still not clear enough.