Build with browserify

I believe this will solve #253.

To preserve our existing test setup, which wants to include individual source files and not require a build step, I used `browserify -r` to build a libs.js that test/index.html includes. This lets `require('mustache')` and `require('reqwest')` work, but requires some maintenance whenever we change dependencies.

It's not totally clear to me how we want to handle the L global. I.e., should `var mapbox = require('mapbox.js')` assign a `window.L` global as a side effect, or no?

cc @maxogden