add @mapperproperty to declarative and document it

[Mike Bayer]

@classproperty should be left alone for now but folks should use a decorator that is specific to declarative's use case, so in future releases we can narrow this down.

[Mike Bayer]

1ff666ffdf1b79433141a006486e21f3c18b1187

I may think further on the name '@mapperproperty' before release....since applying it to __table_args__, __tablename__ etc. are really @tableproperty...but not thinking of other names at the moment.

[Mike Bayer]

Removing milestone: 0.6.5 (automated comment)