Create containing directories when extracting a file

This would cause extractions relying on the new async-io-like
optimizations[0] to fail if the parent directory was not already
created.

[0]: 241fcdf0bd16de9ebbb0f5c57dcb6e5f109480ee
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