between a @declared_attr Column and a
directly-defined Column on a mixin. In both
cases, the Column will be applied to the
declared class' table, but not to that of a
joined inheritance subclass.   Previously,
the directly-defined Column would be placed
on both the base and the sub table, which isn't
typically what's desired.  [ticket:2565]

attributes that are not Column or MapperProperty;
including any user-defined value as well
as association proxy objects.  [ticket:2517]

    single-inheritance declarative subclasses,
    with or without using a mixin, can be resolved
    using a new @declared_attr usage described
    in the documentation.  [ticket:2472]

declared on a single-table inheritance subclass
up to the parent class' table, when the parent
class is itself mapped to a join() or select()
statement, directly or via joined inheritane,
and not just a Table.   [ticket:2549]

the vast majority of file lines are spent on documentation, which moves
into package __init__.  The core declarative idea lives in base and
is back down to its originally low size of under 500 lines.  The various
helpers and such move into api.py, and the full span of string lookup
moves into a new module clsregistry.  the rest of declarative only
refers to two functions in clsregistry in three places inside of base.
- [feature] Declarative now maintains a registry
of classes by string name as well as by full
module-qualified name.   Multiple classes with the
same name can now be looked up based on a module-qualified
string within relationship().   Simple class name
lookups where more than one class shares the same
name now raises an informative error message.
[ticket:2338]
- lots of tests to ensure the new weak referencing memory management
is maintained by the new class registry system.   this ticket was
served very well by waiting to do #2526 first, else this would
have needed to be rewritten anyway.