Cache signature files (.sig extension)

[nroi]

Flexo does not cache signature files. That means if a client requests a package that has already been cached, the package itself can be served from the local file system, but Flexo still reaches out to the remote mirror to fetch the signature file. This introduces a needless latency for serving the package.

Another problem is that features such as #64 cannot be implemented, because Pacman needs both the signature file and the package file to be available locally in order to use a package from the local file system instead of making a new HTTP request.