Check to see if a field is present before using it.

@jamesturk I'm still a bit shaky in this part of the code; was getting:

```python
    if items_differ(jsonsubitems, dbsubitems, subfield_dict[k][2]):
  File "/home/tag/dev/sunlight/pupa/pupa/importers/base.py", line 52, in items_differ
    if k not in subfield_dict and getattr(dbitem, k) != jsonitem[k]:
KeyError: 'organization_id'
```

After this, it's much happier. Unsure what the best bet here is

(Also, now that I look at it, if we have null in both that might break, is there a case where we can get a key that's not present in each? My gut says no)