Use vfs.FileSystem interface instead of vcs.FileSystem.

Fixes #5.

That should be all the references as far as I can find.

However, I see this repo vendors its dependencies. I've never used the `--copy=true` mode of `godep` before, and I tried to get it to work now, but I wasn't sure how to get it to pick up dependencies from all subpackages instead of just the Go package in the root folder... Any ideas?