Refactor source with browserify for browser/server interoperability

This hefty refactor allows the library to run in the browser or on the server. We've replaced grunt w/ gulp, and added a few more tools to handle context-specific builds and tests. Server-side testing is handled with mocha, and browser build are tested w/ PhantomJS, Karma (Chrome, Firefox, Safari), and SauceLabs (iOS, Android, IE9-10).

**Breaking changes:**

* All success+error callback pairs are now handled with a single `function(err, res){}` callback signature.
* `Keen.Visualization` and `request.draw()` have been removed. This is now handled with the `Keen.Dataviz`, which will take center stage in our documentation and visualization guides.