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. 
The problem arises with nested objects. For example when creating a LimitOrder object and then passing it as an argument to create a PlaceInstruction, I get a TypeError.
"TypeError: ModelMeta object argument after ** must be a mapping, not LimitOrder"