Subject wrapper should inherit from unicode, not str.

This wrapper is used in the implementation of clean_subject(). Since all
headers are returned as unicode objects, it will cause a
UnicodeDecodeError if the subject contains unicode and clean_subject()
is called.
Use non-deprecated forms of _bdecode and _qdecode.
Merge pull request #5 from spang/master

Subject wrapper should inherit from unicode, not str.