remove unused instance variable
Merge pull request #76 from databus23/patch-2

remove unused instance variable from CRL::Administrator
Updated r509::CRL::FileReaderWriter.read_list to iterate and yield which removes the
need for a reference to a r509::CRL:Admistrator object.
Update docs to reflect new behavior
Merge pull request #80 from jsuhre/decouple_crl_readerwriter_administrator

Decouple CRL::FileReaderWriter from CRL::Administrator