Fixes Dropbox authentication

Cool, thanks!

We need to focus on the async flow readability a bit. Generally, we have these instruments:

this is specified during runtime based on how the method gets called. Anything executed as a callback will have incorrect this.
func.bind(stuff) will create a bound version of func that will always have this == stuff (independently on how func gets called)
generally speaking, nested callbacks are best solved using Promises. We would have to use a Promise polyfill for older browsers, though.

Let me refactor the current solution using proper binding (without explicit closures) first; I will consider adding Promises later.

ondras / wwwsqldesigner