Fixes issues with the `tf.train.Saver` in the 0.7.0 release.

1. The cifar10_train.py example model was emitting warnings, because of
   non-Variable objects in the `tf.moving_average_variables()`
   collection. This change fixes that by only adding `Variable`-typed
   objects to that collection in `moving_averages.py` (which better
   agrees with the definition in `tf.GraphKeys.MOVING_AVERAGES_VARIABLES`).

2. Saver.save() now calls `tf.gfile.MakeDirs(os.path.dirname(save_path))`,
   which fails if `save_path` does not contain a directory component.
   This change fixes the implementation of `tf.gfile.MakeDirs('')` to be a
   no-op (which better matches the internal library that it is shadowing).

Fixes #1123. Fixes #1135.
Change: 114895020