Add browserify support to make npm module more meaningful

Even when installed via npm, impress.js does not export anything.
It directly decorates "window" which it assumes to be the global variable, even though that's not how it works outside of the browser.

Start by locating the npm entry point and the initialization code that directly decorates "window". Check how the package behaves under browserify and identify the existing test or build command for npm usage. Done means an npm-installed impress.js module exposes a usable export without assuming a browser global.

Written by the indexing model from the issue text.