Cutoff duplicate parts with download builder

Fixes https://github.com/jquery/download.jqueryui.com/issues/122.

Changes summary:
- Modified the `release` task. Now, it uses DownloadBuilder objects to build the release package (which is basically the development-bundle dir of the downloaded package, with a few additions/changes https://gist.github.com/rxaviers/5253603).
- Modified the `generate_theme` task. Now, it also uses DownloadBuilder objects to accomplish that.
- The task `release_cdn` hasn't been changed at all. Well, part of the process is `generate_theme`, which is fully using DownloadBuilder already. But, copying files and md5'ing are still the same.

BEFORE vs. AFTER of `release` (aka pre-release):  https://github.com/rxaviers/jquery-ui/commit/3f85f5c4a91ee1f195de8642b98f3abb8123f978
- JS minified bundle are different. DB uses grunt-contrib-uglify 0.2.0 vs. grunt-contrib-uglify 0.1.1 of the former tasks (although, both uses uglify-js 2.2.5, so I suppose they might use diff parameters);
- CSS bundle are the same. The only diff is that DB do not add a line break between each component styles when merging them to generate the base css. This makes DB implementation slighter easier;
- CSS minified bundle are the same. The only diff is that comments, base css and theme css were all inlined. Now, we have comments \n base css \n theme css;
    images have different md5 (on each build they get a diff md5, so skipping);

BEFORE vs. AFTER of `release_cdn`: https://github.com/rxaviers/jquery-ui/commit/585fe9e8ccbda32229998901ffcd52ec01ee0920
- Virtually, no changes... (a) ignore the images, and (b) note the fact DownloadBuilder had fix improvements, therefore we'll get fixed css banners.

Related changes:
- DownloadBuilder PR: https://github.com/jquery/download.jqueryui.com/pull/115