Read atomic write pipe buffer size from system configuration in piper.py
Merge pull request #62 from socrata/pipe-buf-size

Read atomic write pipe buffer size from system configuration in piper.py
Alphabetize import
Adds a test that creating a TarMemberTooBigError works.
Adds apparent file size to TarMemberTooBigError hint.

Updates tests correspondingly.
Add compatibility with gevent 1.0

A subscriptable sequence is required nowadays to gevent.killall, a
mere sequence (like a set) does not suffice.
Deprecate broken 'setup.py test' integration

Just demand use of 'tox' instead.  Ideally 'setup.py test' would work
smoothly but on account of a couple of problems with program arguments
and plugin discovery, expeditiously kill off the half-working thing
instead.
Install coverage pytest plugin by default

Note the plugin is installed, not used all the time -- it slows down
test execution by quite a bit.  Yet, I have found in practice not
having the '--cov' option at hand makes me less likely to use coverage
in simple cases, so just make it installed by default to put it close
at hand.
Add test coverage reporting instructions
Fix rst warning about too-short title underline
README untabify and alignments
Update setuptools/PyPI authorship metadata
Documents the --terse flag.
Added a new blobstore based on OpenStack Swift
Adds a flag to retain a fixed number of backups.

Doesn't implement the behavior for this flag, though.
Rename delete_if_qualifies -> delete_if_before.
Fixes dead variables in delete_if_before.
Pulls out groupdict_to_segment_number and delete_if_before.
Pulls out a helper function that deletes WAL segments before a cutoff.
Implements "delete but retain N backups" functionality.

- Pulls out a helper function to delete base backups before a
  fixed segment number.
- Implements "delete with retention" using this helper.