Read in static files into `collection.files` as `StaticFile`s.

Fixes #2555.
I'm fine with both implementations. I was actually working on this one (where static files were StaticFiles instead of Documents), but switched over to the Document one as per [your comment in the PR](https://github.com/jekyll/jekyll/pull/2615#issuecomment-49335482) (it was a bit simpler to implement it with Documents as well).