Revocation document generation is stored inside Certification folder

duniterpy/documents/certification.py –> duniterpy/documents/recovation.py

Refactor needed.

So we have 3 classes in certification.py:

• Identity
• Certification
• Revocation

We can put all the classes in a identity.py module (it makes sense, because all classes focus on identities).

Or we can put each class in its own module... May be more explicit.

Please feel free to comment...