Merge Attributes fix

Merge Attributes was breaking interfeatures.  It appears as if merge attributes assumed that each attribute it was merging was a list, when in one case it was a unicode string.  I changed merge attributes to convert all values in the two given dictionaries into a list before merging and to return those lists after the merge.

This pull request might not be optimal because in some cases you end up with a list of length one for common IDs and things.  What should the correct approach be with this?

I've also written up a test covering what I assume is the correct functionality.  

Let me know if you've got questions / want modifications made.