Add Mite::Class->parent_attributes

Allow a class to get just the attributes of its parents.  This will
be handy when implementing attribute extensions.
Let attributes clone each other.

Needed for has '+foo'
Mite::Class->extend_attribute

Implements the guts for has '+foo'.
Fix undef defaults.

It was coming out as an empty string.
Implement has '+foo'
Merge branch 'issue/6'

Implement has '+foo'