dataservice should support unified field for id / desc fields from new knesset API

[davi17g]

@OriHoch:

the new knesset API have a lot of these fields where you have somethingID and related somethingDesc
worth to create a new KnessetDataServiceField class which will handle it automatically -
so instead of defining those two fields, you would define only 1 field:
("type", KnessetDataServiceIdDescriptionField("SubType", "type of the bill"))
behind the scenes this field can expose a python tuple of (id, description)

[davi17g]

see related pull request: #8