Add ability to download jQuery Mobile Theme as a NuGet Package

[mikehostetler]

Worked with Scott Hanselman to extend the jQuery Mobile Themeroller to add the ability to download a NuGet package.

A NuGet package is just a Zip file with some specific contents, so I just copied the existing file that creates a zip package and modified it.

[scottgonzalez]

I don't think we want to favor any specific package format. This opens up a huge can of worms for other environments.