Files in manifest - to be merged *after* pull-request 4

This changes freesite inserts to put the smallest files into a container, up to 2 MiB.

It starts with the smallest files referenced from index.html and includes all files which fit in 2 MiB. Then it checks whether further unreferenced files fit into 2 MiB. Then it adds all further files, starting with the slowest, until there are at most 500 files left out of the container. The rest of the files are uploaded as separate files as before (CHKs).

For graphic-heavy freesites like Freenet_Meltdown, this can reduce the site load time by more than factor 3.
`kw.get('Codecs', ", ".join([name for name, num in self.compressionCodecs]))` is repeated. It seems a good candidate for a function. `get_codecs(kw)`? It returns codecs the user specified, or defaults. Maybe just a `get_default_codecs()` for the default part.