Add support for requesting just the _id and _type of each hit.

Currently, you can only specify at least one field to be
returned, or the whole document.  There is no way to request
that no fields be returned.

Closes issue #26
> So `self.fields()` and `self.fields(None)` would both cause the entire document to be returned, and `self.fields([])` would cause no fields to be returned.

This makes a lot of sense to me and seems intuitive. :+1: