- Added support for @classproperty to provide

- Added support for @classproperty to provide
any kind of schema/mapping construct from a
declarative mixin, including columns with foreign
keys, relationships, column_property, deferred.
This solves all such issues on declarative mixins.
An error is raised if any MapperProperty subclass
is specified on a mixin without using @classproperty.
[ticket:1751] [ticket:1796] [ticket:1805]

- un-anglicized the declarative docs