Sanitize sendmail's to_addrs parameter

This allows sending emails to addresses containing non-ascii characters.

My problem was with code 
```
mail.send_message(recipients=[u'Pekka Pöyry <pekka@fastmonkeys.com>'], ...)
```
that failed with error `UnicodeEncodeError: 'ascii' codec can't encode characters in position 11-12: ordinal not in range(128)` inside sendmail function.