Exclude concepts without fields from the ConceptManagers published set

This change includes a test to make sure that this is indeed the case. I am of the opinion that publishing a concept with no fields does not make sense so I took the easy route here and removed them from the published set. If anyone thinks we should handle published concepts with no fields in a different manner feel free to comment here but I wanted to either close this or get the conversation started.

Fixes #150.