ExtJS 4.2 support

I've updated the Classic theme assets for ExtJS 4.2. The Neptune theme compiles, but gets some image paths wrong in the Sass because Sencha hard-coded them. Le sigh.

This branch also adds the Sencha-provided Ext.ux extensions to the gem. They're not minified, but app developers can require individual files in application.js. The Ext.ux files live in vendor/assets/ux/extjs4-rails/src/ux because Ext.Loader.require() automatically looks there when loading files dynamically.