Use the cryptoconditions spec terminology for public/private keys

Relates to https://github.com/bigchaindb/bigchaindb/issues/746

In BigchainDB we unified the language of keys to public key and private key, instead of using public/verifying private/signing interchangeably throughout the code and documentation.

Cryptoconditions should do the same but use the terminology used on the spec

Start by comparing terminology used across the Python code and documentation with the Crypto-Conditions specification, then inventory occurrences of public/verifying and private/signing language. Done means the project consistently uses the specification's public-key and private-key terminology wherever applicable; no files or tests are named in the issue.

Written by the indexing model from the issue text.