[Alchemy] AlchemyLanguage getCombinedResults call should be able to return extracted dates

[roy713]

According to the documentation of the AlchemyLanguage combined call, the combined call should have the ability to return dates. This can be specified as part of the extract parameters for the call.

A possible solution for this is adding the following to the CombinedResults class:

List<ExtractedDate> dates;

Also, the AlchemyLanguage class should provide constants for other types of extract parameters that are not currently present in the class, such as doc-sentiment, relations, concepts, taxonomy, etc.

Also, there's a showSourceText parameter in the call which returns the text that was passed as an input as part of the result, but I don't think this is part of the CombinedResults class either (you have it as an option in the parameters but I think the data member needs to be added to the class too).