parse marketBook data

[durtal]

One of the methods marketBook returns the unparsed response from Betfair's API. This is due to the response being heavily nested and making life difficult to write a function to organise it into useful data structures for R users to work on.

see lines

The betfair_parse method will likely look like the parse method for marketCatalogue (see here), but I need to explore the data that can be returned.

[durtal]

attempted to organise data here with print and summary methods, returned data includes the raw list as well.

Will likely need improvements over time.