Add a description field for datasets

[goldpbear]

This isn't high priority, but it might be nice at some point to add a field to the Django admin panel that allows us to record a short sentence describing the purpose of each dataset. If we find ourselves needing to create numerous datasets to support featured places with various permissions/access rights, a description field might help us manage the administrative overhead.

Also, this could be a nice foray into building out admin functionality on the api.