Make the theme/css/sdk.css script available in the "source" version of SDK

[wwalc]

#126 came out from the lack of understanding what is the benefit of having full HTML pages as "source" files for SDK samples that are later built. This is because the CSS file, that should be used by the samples, does not exist. It started happening after we switched to sass.

I think we could create a grunt task called prepare-dev that would create the "theme" file locally in the SDK root directory, so that it was possible to work on SDK samples and see how exactly they will look like, without having to build them. The "theme" directory should be added to .gitignore at the same time.