refactor the file system code to return our custom FileEntry objects ins...

...tead of js.Proxy objects

This updates the APIs for fileSystem, syncFileSystem, runtime, and mediaGalleries. I split the wrapping classes out into their own library, lib/src/files.dart.

I also rev'd the version number to 0.3.0-dev; this will be a breaking API change. I'm hoping to push to pub soon, so I can consume the file system changes.