Add DatasetQuery

[chasleslr]

We should create a class that helps with creating queries as expected by the query argument in Dataset.create() (see POST /api/dataset).

This would, at least, serialize to dict (i.e. DatasetQuery(type='query', 'query'={'database': 1, ...}, ...) -> {'type': 'query', 'query': {'database': 1, ...}, ...}), but ideally help with building valid MBQL.