Only call finalize on file closing.

Refactor a bit the logic for creation of cipher, decryptor, encryptor
Merge branch 'feature/cryptography' of github.com:globaleaks/GLBackend into feature/cryptography

* 'feature/cryptography' of github.com:globaleaks/GLBackend:
  various fixes to pycrypto -> cryptography change. to bi fixed

Conflicts:
	globaleaks/security.py