chore(build): make custom builds work (from browser)

Online demo: http://chrisirhc.github.io/angular-ui-bootstrap/

I'll clean up the commit messages in a bit.

Most of this is based on Twitter Bootstrap's in-browser custom build logic.

What's left:
- [x] Show message when browser doesn't support custom builds
- [x] Speed up initial page load by loading custom build JavaScript files last

Fixes #2960, #2847, #2625, #2489, #2357, #2176.