link to Modoboa in README
ensure Python operates in UTF-8 with a consistent locale for all users

fixes #206 (hopefully)
dns_update: assume DKIM is present
add strict SPF and DMARC records to any subdomains (including custom records) that do not have SPF/DMARC set

closes #208
Merge branch 'master' of github.com:mail-in-a-box/mailinabox