Transform field names on related collections before merging

When flattening data for baking, transform the field names
before merging them together.  This means more method calls, but
results in cleaner code.  It also lets us get rid of the dotted
data field convention (e.g. ``contest.start_date``) in the
internal representation and the final output.

Right now there aren't any field name conflicts between models
that are getting merged together.  If there are in the future,
just define a ``FieldNameTransform`` to avoid clobbering a field
contributed by another model.

Addresses #39