Allow renderjs from JS only, without initial gadget-tag/script-tag

Genereal

• In order to integrate renderjs and jupyter I need to be able to load everything-renderjs from a javascript interface. The inital loaded HTML does not contain anything of renderjs and gets added only after pageload.
  I thus added functionallity to renderjs to initialize manually, after pageload. See the forum entry here.

• Instead of registering init() for the DOMContentLoaded event directly, a deferred promise is used which gets resolve either by the event, or by a manual call of rJS.manualBootstrap().

Tests:

• Injects rsvp/renderjs in a blank iframe
• makes sure renderjs is only present after it was injected
• calls manual bootstrap to initialzed renderjs
• creates small test gadget to ensure renderjs is working within the iframe (after bootstrap was executed)