PGPSignature is alive again! - #80
 - Two property methods are not yet because I don't have test data for them yet
 - Added __sig__ method to Signature(MPIs). DSASignature.__sig__ outputs in (hopefully) correct ASN.1 DER format
 - Removed keyfields.py finally
 - Changed expiration subpacket payloads to timedelta objects (as they should have been already)
 - Changed SignatureV4.hleft to SignatureV4.hash2 for consistency
 - Added unit test harness for basic loading of PGP blocks
 - Added unit test harness for loading/testing signature blocks
 - Added unit test stub for (eventual) loading/testing key blocks