build mocha.js using browserify

first attempt to building mocha.js with browserify instead of all the custom stuff.

- [ ] i'm not sure what `support/tail` does, but i haven't incldued it in the build. still need to do that.
- [ ] the previous mocha `require()` didn't include mocha's dependencies, but browserify does, making the build too big. not sure how to disable them all without wanting to kill myself. i'm thinking just to create a separate browser entry point that doesn't require all the reporters