crash after recent commits: assert manifold_w * manifold_h == num_images

Got problem while saving the samples:

```
Traceback (most recent call last):
File "main.py", line 97, in
tf.app.run()
File "/usr/local/lib/python3.5/dist-packages/tensorflow/python/platform/app.py", line 48, in run
_sys.exit(main(_sys.argv[:1] + flags_passthrough))
File "main.py", line 80, in main
dcgan.train(FLAGS)
File "/home/prota/Downloads/DCGAN-tensorflow-master/model.py", line 298, in train
save_images(samples, image_manifold_size(samples.shape[0]),
File "/home/prota/Downloads/DCGAN-tensorflow-master/utils.py", line 249, in image_manifold_size
assert manifold_w * manifold_h == num_images
AssertionError
```
manifold_w == 8 while manifold_h == 64. should be both 8 if images are 64?

Read model.py around the train function's save_images call, then inspect utils.py:image_manifold_size and the reported manifold dimensions. Reproduce the sample-saving path from main.py; done means the assertion passes with the intended manifold dimensions and samples are saved without the crash.

Written by the indexing model from the issue text.