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. 
According to the official docs, `selectionId` is supposed to be a `long` (https://api.developer.betfair.com/services/webapps/docs/display/1smk3cen4v3lu3yomq5qye0ni/Betting+Type+Definitions#BettingTypeDefinitions-Runner). Are you running into `selectionId`s that are strings? Can you give an example?