Fix PEP8 errors and warnings.
Add default_sender_address to the settings.
Add sender to the model and the form schema.  Use sender when sending emails.  This prevents problems w.r.t. SPF.
Add a migration script.
Update docs.
Get default_sender from the .ini files' mail.default_sender option if not set in the settings for the site.
Adjust test.
Update translations.
Test against target version of Kotti that matches the requirements.txt installed before.
Merge pull request #11 from Kotti/feature/reply_to_header

Use user specified email address in Reply-To header instead of sender