documentation updates
Handle unset cert_spec.

For PKCS#11 key_spec, cert_spec may well be empty.
Fix PKCS#11 URL keyspec.

With PKCS#11 URL, the public key may be unknown and should
not be parsed (would fail).
Keysize fix for PKCS#11 URLs.

The keysize is only needed for padding, and padding is not done
in this module for PKCS#11 signs anyway.

Don't require it to be present in the 'private' dict when it won't
be used anyways.
sign: Fix expectations with PKCS#11.

With a PKCS#11 key_spec, there will never be a private['data'].
Merge pull request #14 from fredrikt/master

Some PKCS#11 fixes.