Updated address length check to 1024 characters because many unsubscribe
links were much larger than our old limit (which was already 4x the RFC limit).
New limit is 16x the RFC limit.

Bumped the version of dnsq and expiringdict.

Moved mock and nose from install_requires to tests_require.
Make FallbackMimePart interface match with MimePart

To achieve that common functionality has been extracted to
 RichPartMixin that is mixed to both classes.