Add shortcut `Dataset.filter()` to avoid confusion with the `.query()` method

[davecap]

The requirement to call Dataset.query().filter(field=value) can be confusing. We can simplify this for most use cases by adding a filter() shortcut method to the Dataset object in the case where you do not need to provide additional query parameters. This would just call self.query().filter(*args, **kwargs) on the dataset instance.