Remove requirejs, replace with browserify

requirejs and browserify can't co-exist[1]. The tutorial is full of requirejs stuff that needs to be reworked to be browserifyable. I didn't want to do that as part of this PR because word on the street was the tutorial was going to be re-written anyway.

[1]: Many modules are published with [UMD preamble](https://github.com/umdjs/umd) to be compatible with multiple modules systems. UMD only expects one module system to exist on a page, so if it detects requirejs, it will not properly export commonjs modules.