Print command at start-up with more attention paid to rendering

In particular, 'delete' has two levels of subcommands and knowing
which one was used is handy.
Use s3-external-1 endpoint for us-standard

This is the endpoint in us-east-1, where most instances are.  This is
not being applied to the v0.6 maintenance releases to avoid changing
behavior for those who are closer to Seattle.
Elide other backends besides S3 in requirements.txt

It's not clear how to make things cleanly optional but not a usability
nightmare (e.g. installing *no* backends by default for 'pip install'
based people) within the Python packaging framework, so don't try..for
now.
Enable S3 server-side encryption
Avoid deleting keys that have a new-ish timestamp

This is a paranoid safeguard against mess-ups; even if turned against
the wrong prefix it is likely many backups will be retained.

Some of the tests need adjustmenmts to not freak out about this
safeguard.