Better admin output for email text and subject. Fixes failures for emails with Subject/From/To fields missing.

e.g.:
echo "Hello there" | mail test@yourdomain.com
would result into a missing email subject.

Needs some rework for get_filename() method.
Not tested most of the changes by @Anders.Du : addresses, filename parsing updates and __getitem__ removal.
What works well: 
subject parsing, text body parsing, missing fields fixes.