Allow only primary fields in the column selector

Child fields were not explicitly kept out of the table view, only by
convention of not giving them a "TableDisplayName".  The client
did not follow that convention, so added an explicit check to prevent
any child resources from becoming an option in the col picker.

Fixes #86
Merge pull request #90 from azavea/topic/program-name-attr

Allow only primary fields in the column selector