Fixing Batch class docstring (referenced dead code).

Also adding Dataset class as a documented import in the
datastore package __init__.

Fixes #709.

gcloud/datastore/__init__.py

@@ -28,6 +28,10 @@
   which represents a connection between your machine and the Cloud Datastore
   API.
 
+- :class:`gcloud.datastore.dataset.Dataset`
+  which represents a dataset ID (string) bundled with a connection and has
+  convenience methods for constructing objects with that dataset ID.
+
 - :class:`gcloud.datastore.entity.Entity`
   which represents a single entity in the datastore
   (akin to a row in relational database world).
@@ -56,6 +60,7 @@
 from gcloud.datastore.api import put
 from gcloud.datastore.batch import Batch
 from gcloud.datastore.connection import Connection
+from gcloud.datastore.dataset import Dataset
 from gcloud.datastore.entity import Entity
 from gcloud.datastore.key import Key
 from gcloud.datastore.query import Query

gcloud/datastore/batch.py

@@ -51,8 +51,6 @@ class Batch(object):
 
     By default, no updates will be sent if the block exits with an error::
 
-      >>> from gcloud import datastore
-      >>> dataset = datastore.get_dataset('dataset-id')
       >>> with Batch() as batch:
       ...   do_some_work(batch)
       ...   raise Exception() # rolls back