setup.py: Use __url__ and __author__ from rss2email/__init__.py

Instead of duplicating those values locally.  This gives us one less
place to forget to update the next time we change the website or
maintainer.

Also, update __url__ to point to my GitHub repository and shift the
email address to rss2email.__email__.

Signed-off-by: W. Trevor King <wking@tremily.us>
error: Import __*__ metadata (URL, author, email)

These are used to format some of the log messages.

Signed-off-by: W. Trevor King <wking@tremily.us>
Bump to version 3.1

Changes since 3.0:
* Import __url__, __author__, and __email__ in rss2email.error, which
  fixes bugs formatting a number of errors.

Signed-off-by: W. Trevor King <wking@tremily.us>
CHANGELOG: Update after the release of 3.0 and 3.1

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