Decouple CRL::FileReaderWriter from CRL::Administrator

I'm evaluating r509 for use as a CA. On of the items I'm working on is implementing a DB CRL backend instead of the default flat files. When looking at the code I thought it would be better for FileReaderWriter to have no knowledge of the Administrator object. So I updated FileReaderWriter to yield instead of calling Administrator.revoke_cert. This should make read_list usable for other purposes and allow for alternate implementations of CA::Administrator

Feedback always welcome!