Let SPSA attack be called using generate_np

[nottombrown]

I spent some time this afternoon trying to get SPSA to hook into the generate_np harness, but had trouble getting it working.

Currently the SPSA test works correctly using the sess.run syntax, but it would be slightly nicer to have it be callable with generate_np

https://github.com/tensorflow/cleverhans/pull/498/files#diff-ee72988b849d43f34dfd258c5f95fc8dR270

I also think that it would be good if SPSA could be called without any explicit labels (the default behavior for other classes is to use the models labels on the clean data as the label to attack in that case.

[nottombrown]

cc @juesato

[goodfeli]

This works now, maybe since #509?