Remove unused lib/app.build.js and optomist library

The only references to them are the initial commit
541af0bd473ffe154557b5f978ad25c2b1c4c38

this moving of the file:
16ad49aa670fa1207f873beb0069c5575be77485

and this cleanup that says "I don't know what this file is for":
2906bc78cf765e7b91fc00dc5de109265aa50d09
Move sauce-browsers.json to test/

It's used only by tests, so put it in test.
Move src/generate.js to lib/generate-init.js

Generate is not actualy part of the modernizr source, but part of the build
system. The code it generates is part of the source. Adding -init to follow the
same pattern as generator-meta, and because it's more descriptive.
Switch generate-init to node-style module

Keeping build code node-style, and Modernizr output code require-style.