Removing cursor and more results from query object.

This is a follow up to a discussion in #423 and #425.

Also
- Adding `fetch_page()` method to return the needed properties
  no longer available on the `Query` object.
- Made `fetch_page()` fail if `more_results` is `NOT_FINISHED`, which
  seems to be intended for batched requests.
- Added `start_cursor` and `end_cursor` properties to Query.
- Updated docstrings to reflect change
- Updated tests to ensure the returned cursor was always set.
- Streamlined `fetch()`/`fetch_page()` tests to run via a single
  tweakable helper.
- Updated a `datastore.connection` test to construct and return an
  initialized `QueryResultBatch` to simulate a valid cursor.
- Updated `regression.clear_datastore` and `regression.datastore` to
  reflect changed API surface.
From the [spec][1]

```
message Query {
  // The projection to return. If not set the entire entity is returned.
  repeated PropertyExpression projection = 2;

  // The kinds to query (if empty, returns entities from all kinds).
  repeated KindExpression kind = 3;
```

[1]: https://github.com/GoogleCloudPlatform/google-cloud-datastore/blob/21e029b2657ebb15fef5429c90c4315326855fd1/proto/datastore_v1.proto#L209