S3 backend and some Win32 cherry-picks from upstream#28

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willkelleher wants to merge 5 commits intojedbrown:masterfrom

PersonifyInc:s3-backend

Commits on Apr 7, 2014

ensure_binary_mode: python3 and python2/win32 compatibility

jedbrown
authored and
Will Kelleher
committed
Abstract the backend and add S3 implementation
Will Kelleher
committed
encoding: use uninterpreted bytes whenever possible

jedbrown
authored and
Will Kelleher
committed

Commits on Apr 8, 2014

Continue to function if boto is not installed
Will Kelleher
committed

Commits on Apr 9, 2014

s3: Try to use environment variables if s3.{key,bucket} are undefined
dlin
committed