Sign multiple files

[guillomovitch]

When signing multiple files with the same key/certificate pair (for instance, the 8132 files resulting from splitting edugain metadata into mdq ready-to-serve individual metadata files), calling xmlsec.sign() once par file results in a waste of resources, as the key and the certificate are read everytime.

This PR tries to address this issue with a Signer object, instantiated once, used multiple times, such as: signer = xmlsec.Signer(keyspec, certspec) output = signer.sign(input)

I'm not a Python expert, so the code can certainly get improved, but provided tests ensure at least it works as expected.

[leifj]

hey - can you look at the conflicts?

[guillomovitch]

Done, here is a cleaner version.