abstracted the various key types into their own classes/subclasses for better organization and reduced code reuse
subkey access refactoring; almost complete
deleted some commented stuff; added a function to list subkey fingerprints
test harness stuff
moved secret key material decryption to packet.packets: closes #60.
moved SignatureVerification to pgpy/types.py; created new collections object with aliasing for selecting keys and subkeys with via multiple methods. closes #67, #59
updated tests
PEP 8