Add support for querying for alternatives in property values

[mwatts15]

DataObject properties currently have an implicit "AND" for queries: all values associated with the property have to be in the graph for a match. It would be good to have an ability to query for alternative values -- to have an "OR"-type behavior. This would allow for fewer queries to be made in several cases: currently you'd just have to make the query multiple times with different values set or else drop down to a Graph.triples_choices()