Allow jQueryUI to generate its release files/package using DownloadBuilder

Addresses #122.

Changes summary:
- Drop requirement for a jQuery UI release, instead use a jQuery UI build. The former preparation step required running `grunt release`. So, it would run into a circular loop when jQueryUI uses DB to generate its releases;
- Make DownloadBuilder a low level tool by exporting its main classes. The former API exported a highlevel few specific functions. Now, we export the building-blocks (classes) necessary to allow jQuery UI (the client-application) to do whatever it needs to build its release files using the abstraction of our classes;
- Added options on Builder to allow it to generate the needed files for a release as well as the downloaded packages the smoothest way;
- Several minor fixes, including banner fix and the addition of demos/index.html dynamically generated to conform with the selected package components;

Downloaded package BEFORE vs. AFTER: https://github.com/rxaviers/download.jqueryui.com/commit/463a46c27a8df85e5ebf185f9b76354988d88720
- PS: ignore the images

Related changes:
- jQuery UI PR: https://github.com/jquery/jquery-ui/pull/951
- jqueryui.com PR: <wip...>