Replace top-level this keywords with window for usage with CommonJS and Browserify

I tried using foundation in combination with [browserify](https://github.com/substack/node-browserify), however I noticed that all modules use `this` at the top level. 

However, when using CommonJS modules, which browserify does, `this` at the top level refers to the current `module.exports` object, instead of the `window` object. So unfortunately foundation will no longer load correctly in that context.

By replacing `this` with `window`, which has no drawbacks as far as I am aware, this problem is solved, and foundation can be used in a CommonJS module format as an additional benefit.