Bug 1003876 - Derive bookmark title from web manifest

OK, I wrote a version which used LazyLoader.getJSON, but in doing so I remembered that we should be enforcing a Content Type of application/manifest+json to be consistent with what Gecko is doing and to meet the spec. I also remembered that eventually we're going to need to pass another new argument to the XHR constructor to do a special kind of XHR which allows you to get a resource using a particular browsing context (e.g. for HTTP authenticated manfiests).

Overloading LazyLoader.getJSON with all of this seems to be going a bit far as it's really designed just for fetching local resources from the system app as far as I can tell. I think we should maybe keep this separate, or in future write a new getJSON helper for getting remote resources if this kind of thing is done in multiple places. So until then I added the content type check to ManifestHelper instead, see what you think.