[development] replace drupal.js with prototype.js?

[Jeremy Epstein]

Prototype does indeed seem to be a very light, clean, and extensible
library: but nonetheless, it is nowhere near as light as drupal.js.
Quick comparison:

Drupal.js: 281 lines, 6.21KB
Prototype.js: 1039 lines, 27.29KB

I am confident of the quality and the benefits of the prototype
library. But we really need to consider whether or not these benefits
can justify quadrupling the size of our base JS file. According to
websiteoptimization.com, web sites should be aiming for a to have 8KB
or less of JS on any given page. This will not be possible with
prototype.

Jaza.