catacomb/__init__.py: Don't print secret bits of keys by default.

catacomb/__init__.py: Don't print secret bits of keys by default.

Introduce a `PRINT_SECRETS' flag which can easily be twiddled (e.g., in
IPython) to control whether obvious secrets are printed literally or
censored (the default).  This is intended to make accidental leakage a
bit less likely, rather than as a security feature.