Bundle citeproc-js and generate citations in-house

Not all citation styles are supported by citeref and datacite. Instead,
use their application/citeproc+json content type to retrieve citation
meta and generate the citation in the extension, using citeproc-js.
Update underscore.js to 1.7.0
Update QR generator

* Use new jQuery.qrcode
* Enable foreground and background color selection using Spectrum
  (version 1.5.1)
* Always use PNG image type
* Allow manual title insertion
Reorganize external resources

Put external resources into their own folders.
Update bubble meta images
Version 2.3.0