External download builder

Abstract DownloadBuilder core functionalities below.
- Handle check/uncheck of dependencies/dependents;
- Handle toggleAll logic;

The code responsible for those has been [encapsulated away](https://github.com/DownloadBuilder/download-builder/blob/master/src/download-builder.js). Our jQuery UI Download Builder doesnt need to handle with that anymore, but instead use a very [simple API](https://github.com/DownloadBuilder/download-builder/blob/master/demo/app.js).

More about it: http://downloadbuilder.github.com/