Warn user in case of missing title. Fixes #440.

When a WP XML file is imported, items with missing title are generated with a
title which is probably not the good one (instead of being dropped), and a
warning is displayed to the user.
remove unittest2 and fix various warnings in py3
remove warning for FEED_DOMAIN when SITEURL undefined
Merge pull request #841 from Rogdham/pelican-import_wp_no-title

Warn user in case of missing title. Fixes #440.
Merge pull request #842 from avaris/remove-unittest2

remove unittest2 and fix various warnings in py3
Cleanup tox and remove mentions of webassets.

Nose and unittest2 have been removed from pelican's dependencies, and webassets
moved to pelican-plugins.
Merge pull request #843 from saimn/cleanup-tox

Cleanup tox and remove mentions of webassets.
Add CONTRIBUTING file; include in contribute.rst

GitHub recently added a feature that looks for a CONTRIBUTING file in
repo root and displays it whenever a user creates an issue or submits a
pull request. I took this opportunity to put some contribution
submission guidelines into that file, including it dynamically inside
our existing docs/contribute.rst file to eliminate unnecessary
redundancy.