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
If `fields` just took a list of fields instead of `*args` I think it would simplify and also fix this issue. It's also a more direct equivalent to what is provided to Elasticsearch.

The method definition could be:
```
def fields(self, fields=[])
```

Which would allow both `Search().fields()` and `Search().fields(['id', 'title'])`.

@HonzaKral How do you feel about the backwards compatibility of the above? Is it too late to change it?