Elements and comparator SQLAlchemy 0.9 compatibility

SQLAlchemy's [changelog for version 0.9](http://docs.sqlalchemy.org/en/latest/changelog/changelog_09.html) notes that the `sqlalchemy.sql.expression` module has been refactored: some objects that geoalchemy2 uses have been moved, or not been exposed in a new module `__all__`. I've therefore updated geoalchemy2's imports to be compatible with both SQLA < 0.9 and >= 0.9. Tests pass on both 0.8.2 and 0.9dev.