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. 
I get a long stacktrace from the parsing of the response to a list_current_orders() request.
The last line is   
File "/home/kai/betmaster/betfair/meta/datatype.py", line 20, in unserialize
    return self.type(processed)
TypeError: \_\_init\_\_\(\) takes exactly 1 argument (2 given)

The value here is a dictionary containing price and size information.