minor changes
documentation updates; custom documentation extension for progress checklists
stub classes for PGPSignature and PGPKey
more test stuff
refactored MPI field classes to:
 - store key parts as integers #94
 - private MPI classes now also store the public key fields - #92
 - do the work to set up the Cryptography public/private key objects. Private key field objects can also return public key objects, so this is part of what is needed for #92
 - updated unit tests to work with the refactored code