Add unit tests about tensorflow-wavenet

[multivac61]

I have started implementing unit tests - https://github.com/multivac61/tensorflow-wavenet/blob/unittests/test.py

@lemonzi has suggested that code that is independent on hyperparameters (_preprocessing etc) be moved outside the WaveNet class. That makes testing on them easier and it leads to better code structure.

[lemonzi]

Yep! I think we could move out _preprocess (which is actually encode), decode, dilated_conv, and maybe something more.

[ibab]

Yeah, we can create a wavenet_ops.py or just ops.py file that we can move all independent functions to.

[ibab]

I've added a first unit test at test/test_causal_conv.py.

[lemonzi]

It seems we have decent unit tests for now. Pieces not covered yet should be listed in Projects or in the Wiki.