Signing time verification

[KeyvanArj]

In verifier.py module, there is not any verification for signing_time attribute included in cms. I've added it. Would you like to add it to the module?

[m32]

just make PR

The purpose of endysive is not to verify the signature, but to sign the document. Verification should cover much more than what endesive offers today, e.g. verification of a time server certificate, CRL lists, ...