Add a ResultSet.find() method

It would be nice if ResultSet had a find() method that behaved similar to the one on bound ReferenceSets. That is:

 1. takes a list of where conditions that are used in addition to whatever conditions are already in place, in essence narrowing an existing search.
 2. returns a new ResultSet.

This would allow repeated filtering of result sets in a fashion similar to what Django allows, for instance.

Some things that we would need to be careful of:

 * sliced result sets.
 * set expressions (union, intersect, etc)
 * aggregate queries (i.e. ones using group_by()).
 * any of the above for finds on multiple tables.

Raising FeatureError for some of these might be a good starting point.