sop.md, sop.py, and sopgpy improvements

Here are several improvements to sop at all three levels: documentation, python framework, and PGPy implementation.

Documentation (sop.md):

• added armor and dearmor subcommands
• consolidate documentation about return codes

Python framwork (glue/sop.py):

• more pythonic arrangements (enums, exceptions, etc)
• simplified extensibility for implementers who want to add simple options
• added armor and dearmor subcommands

PGPy implementation (glue/sopgpy):

• do not try to encrypt to IDEA even if that's the only matching cipher
• fix subkey usage flags
• encrypt -- use provided session key
• implemented armor and dearmor subcommands
• version information: include version of sop as well as the version of PGPy