A few bugfixes

- fixed a few typos in models.py
- matched_date field in the definition of ClearedOrderSummary should not be required because it is not available in responses to some requests
- meta/datatype.py: accept a dictionary as argument for object initialization
- datatype.py: the old code caused an error where some unserialized object was accessed with **value. 
This looks reasonable, but is there a bug that it fixes?