declarative mixins

[Chris Withers]

Please merge this branch:

http://svn.sqlalchemy.org/sqlalchemy/branches/chrisw_mixin

Which has:

• implement the declarative mixin pattern

• change one use of cls.dict to dict_

• tweaks to quieten down sphinx about what it thought were errors

• re-shuffle of the declarative API docs

• slight reshuffle of declarative's mention in the orm tutorial

• link to declarative docs from the orm tutorial

[Mike Bayer]

this branch is fantastic. committed in f60a5fbd5d52c74a33441ea834ede839584595fc.

[Mike Bayer]

Removing milestone: 0.6.0 (automated comment)