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. 
You're right about that one, selection_id should always be of type long according to the docs.
(It should however also be long in the definitions of RunnerCatalog and ClearedOrderSummary.)

I encountered no problems either way, the type information probably gets lost in the JSON request.