Use ST_GeomFromEWKT

This fixes bugs reported by @pgiraud on the [mailing list](https://groups.google.com/d/msg/geoalchemy/HHu3P0zc1_A/Xz_-HgRbXuUJ).

With this patch the following works:

```
        conn.execute(Lake.__table__.insert(), [
            {'geom': 'SRID=4326;LINESTRING(0 0,1 1)'},
            {'geom': WKTElement('LINESTRING(0 0,2 2)', srid=4326)}
        ])
```