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.
Do not generate invalid filenames. Fixes #814.

Turn invalid characters into underscores, remove leading dots and enforce
a maximum length. Should be fine on main file systems used by Windows, Mac OS
and Linux.
Thanks to @Avaris for helping to clean my code.
Minor improvements to contributing guidelines
Update FAQ and changelog
__import__ fails for unicode 'fromlist' in Py2.x
Merge pull request #849 from avaris/plugin-path-unicode

__import__ fails for unicode 'fromlist' in Py2.x
Output sources for translations as well.
Merge pull request #844 from Rogdham/pelican-import_invalid_characters

Do not generate invalid filenames. Fixes #814.
Merge pull request #846 from Rogdham/output_sources_translations

Output sources for translations as well.
Clarify docs regarding pages. Fixes #847.