- Query has add_entity() and add_column() generative methods. these

- Query has add_entity() and add_column() generative methods.  these
will add the given mapper/class or ColumnElement to the query at compile
time, and apply them to the instances method.  the user is responsible
for constructing reasonable join conditions (otherwise you can get
full cartesian products).  result set is the list of tuples, non-uniqued.
- fixed multi-mapper instances() to pad out shorter results with None so
zip() gets everything