Retire DownloadBuilder-hijack-way of building-demos about jqueryui.com

[scottgonzalez]

This just bit us again. See #32.

[jzaefferer]

For the build-demos task we should just clone jQuery UI again instead of hijacking DB.

Since DB also has a bunch of git-related tasks, maybe we can build a little helper with we can include as a module in both DB and this repo's grunt file.

[jzaefferer]

Scott actually built a tool around git, though focusing on "a bunch of functions that make writing custom git commands or other git-based scripts much easier": https://github.com/scottgonzalez/node-git-tools

Could have something like Repo.fetch() which would handle cloning or fetching based on whether the repo exists.

[jzaefferer]

We can do a flat clone, since we don't need the full history to generate the demos. Just need the revision we're putting on the site. Use the --depth <depth> option to load only the specified number of revisions.

[jzaefferer]

This still sucks.