Bump to version 3.0

Changes since 2.71:
* State storage split into a static configuration file (usually
  `~/.config/rss2email.cfg`) and a dynamic JSON data file (usually
  `~/.local/share/rss2email.json`).
* The static configuration file is parsed with Python's ConfigParser
  class, which allows for default settings that can be overridden on a
  global or per-feed basis.  You'll have to translate your old config
  to the new format by hand when you upgrade.
* Emailed messages now have Message-IDs.
* Feeds can be indexed by name as well as index (e.g.
  `r2e run my-feed`).
* Restructured as a package with submodules instead of a single
  module.  This makes dependencies between various portions of
  rss2email more explicit.
* Converted to Python >=3.2, for more consistent Unicode handling,
  exception chaining, and argparse (although argparse is also in 2.7).
* Packaged with setup.py and distutils, in case you want to install
  rss2email instead of running it from a Git checkout or unpacked
  tarball.
* Added a test suite (run with `./test/test.py`).
* Added a man page, based on the version in the Debian package.
* Require Signed-off-by lines in new commit messages, following the
  Linux and Git projects.
* Assorted cleanups and bug fixes.

Signed-off-by: W. Trevor King <wking@tremily.us>