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

• 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().

• An additional change is required to prevent renderjs bootstrapping multiple times when first loaded via $.getScript, and later included in a gadget via <script> (Line 685). I am not sure if this is the best way to do things though and if it breaks other stuff.

• I am generally new to this library and RSVP so this might be flawed and so, please comment. @romain @klaus