Add capability to send pre-compressed js & css files if the browser can accept them

- save a gzipped version of js & css files at the same time as the uncompressed version is cache
- basic content negotiation to send the compressed files
- uses sendfile (for compressed or uncompressed versions) if config indicates its available

darcs-hash:20090118184354-f07c6-66c5b465ab147d83de409708bab2c47d1dafcf8d.gz