Make logging at debug levels more convenient

Previously, even though MINIMUM_LOG_LEVEL could be set, if set to a
severity like "DEBUG", log entries would be ignored anyway because of
the hard-coded setLevel calls with logging.INFO.

Instead, set the severity from MINIMUM_LOG_LEVEL so that the former
does the more obvious thing when changed to a value like DEBUG.

Reported by Greg Stark <stark@heroku.com>
Support late-binding of log levels

Previously, the early calls to setLevel in configure would be
impossible to un-do, which makes modifying logging behavior from any
kind of user provided option a pain.

Fix this by tracking the handler references added to the logger and
mutating them at a caller's request.  All these steps are encapsulated
and indirected through the procedure set_level.

This obsoletes the old MINIMUM_LOG_LEVEL module global assignment
approach that was introduced for this reason, but unfortunately
obsoletes a test that was easier to write with that approach as well.

Reported by Greg Stark <stark@heroku.com>
Require use of S3 server side encryption

So far, it does not appear as though it has any obvious performance or
usability impacts.
Add Raanan Raz as a contributor
Remove non-deterministic test case

This would sometimes fail on Travis-CI giving spurious failures, and
doesn't test a vitally distinct path.
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.
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.