global: libraries can be loaded from their CDN

• Updates MathJax to 2.5.

• BETTER Enables external JavaScript libraries to be loaded from their CDN.

• Example for MathJax: override the base jquery bundle and set the URL of the CDN to the MathJax entry. E.g. in your overlay: ` from invenio.base.bundles import jquery as _jquery _jquery.bower["MathJax"] = "//cdn.mathjax.org/.../MathJax.js"
  1. of with a fallback: #_jquery.bower["MathJax"] = ("//cdn.mathjax.org/.../MathJax.js",
  2. "~2.5") ` The fallback is sometimes useful if this library has declared dependencies in bower. The best example of that is jQuery.

Signed-off-by: Yoan Blanc <yoan@dosimple.ch>