Added a couple util methods to FixtureDataItem

I think the first method (`get_item_list`) is definitely useful - it lets you avoid looking up the data type, just pass the tag.

The second method `get_key_value_map` is useful for what I'm currently working on, but I'd welcome some feedback on its general utility.

    >>> from corehq.apps.fixtures.models import FixtureDataItem
    >>> FixtureDataItem.get_item_list('opm', 'birth_spacing')
    [FixtureDataItem(_attachments=None, _id='412eb5220c757e08355e9c259b81886d', _rev='4-f8ddde9e97ee2de7e6aa598818589c30', data_type_id='2a84957505efa22f5fdaea9a1cf56e88', doc_type='FixtureDataItem', domain='opm', fields={'Amount (Rs.)': '20
    00', 'Time Difference': '2 years'}, sort_key=None), FixtureDataItem(_attachments=None, _id='ce608ae75e60283b939bb32be19ad25a', _rev='4-6bc01ddc03f9451fc9a79ed609570de7', data_type_id='2a84957505efa22f5fdaea9a1cf56e88', doc_type='FixtureD
    ataItem', domain='opm', fields={'Amount (Rs.)': '3000', 'Time Difference': '3 years'}, sort_key=None)]
    >>> FixtureDataItem.get_key_value_map('opm', 'birth_spacing', 'Time Difference', 'Amount (Rs.)')
    {'3 years': '3000', '2 years': '2000'}