Introduces
- concept of `implicit` for `_DefaultsContainer`.
- testing module for datastore for patching

Verified all test cases that would accidentally rely on _DEFAULTS implicit
behavior by using a custom `_DefaultsContainer` that raises when the
instance is implicit.

Main delta for the custom properties:

diff --git a/gcloud/datastore/_implicit_environ.py b/gcloud/datastore/_implicit_environ.py
index 3afc954..1d38562 100644
--- a/gcloud/datastore/_implicit_environ.py
+++ b/gcloud/datastore/_implicit_environ.py
@@ -38,9 +38,37 @@ class _DefaultsContainer(object):
     :param dataset_id: Persistent implied dataset ID from environment.
     """

-    def __init__(self, connection=None, dataset_id=None):
-        self.connection = connection
-        self.dataset_id = dataset_id
+    class FooError(Exception):
+        pass
+
+    @Property
+    def dataset_id(self):
+        if self.implicit:
+            raise self.FooError
+        return self._dataset_id
+
+    @dataset_id.setter
+    def dataset_id(self, value):
+        if self.implicit:
+            raise self.FooError
+        self._dataset_id = value
+
+    @Property
+    def connection(self):
+        if self.implicit:
+            raise self.FooError
+        return self._connection
+
+    @connection.setter
+    def connection(self, value):
+        if self.implicit:
+            raise self.FooError
+        self._connection = value
+
+    def __init__(self, connection=None, dataset_id=None, implicit=False):
+        self.implicit = implicit
+        self._connection = connection
+        self._dataset_id = dataset_id

 def app_engine_id():