3.1.10 CRL objects 
CRL objects have the following methods:
- add_revoked(revoked)
- 
Add a Revoked object to the CRL, by value not reference.
- export(cert, key[, type=FILETYPE_PEM][, days=100])
- 
Use cert and key to sign the CRL and return the CRL as a string.
days is the number of days before the next CRL is due.
- get_revoked()
- 
Return a tuple of Revoked objects, by value not reference.