Feature names inconsistent

[ahwagner]

All records from BRCA exchange have a list under the feature_names attribute, other sources have a string or None object here. The list is never longer than one item, so I propose we fix the harvester to assign only string or None objects to the feature_names attribute.

Omitting the review label because this is a straightforward, quick fix.

[bwalsh]

Can we combine this with #88 ?
( affects the same area of the code )

[bwalsh]

done 2fd2e8e