check if ColumnProperty really contains a Column

SQLAlchemy allows to explicitly use the column_property function (http://docs.sqlalchemy.org/en/rel_0_9/orm/mapper_config.html#using-column-property-for-column-level-options) which will generate ColumnProperty objects which not necessarily contain Column objects but e.g. SQL queries. ColanderAlchemy implicitly asserts that a ColumnProperty contains a Column. By explicitly checking for their existence the implementation is more robust and doesn't crashes with
ColumnProperty(s) that aren’t Columns.
PS: Sorry my editor fixes whitespace, if this is a problem I will see if I can do the changes in another editor.