Updated file asset type

No longer is there a Root File asset. Now File assets will copy to a
site much like page assets. The path will match the location in the
files directory. so plugin_assets/files/robots.txt will end up in
_site/robots.txt.

Much like page assets, files will match their plugin path and can be
installed anywhere unless there is a file in that location. Also files
in _plugins/files/whatever will overwrite the plugin path.