[WIP] Reimplement beam visualization

[bpopeters]

This pull request reimplements dumping a json file representing the beam, as was requested in #719. It's not perfect, however:

1. Beam visualization only implemented with the slow translation path. Not such a big problem because you're unlikely to want to visualize more than a few sentences.
2. It's only implemented with batch_size == 1. With larger batch sizes, the beam file generates but in scrambled order and I'm not sure why. As above, not such a big deal.
3. Occasionally I'm getting indexing errors when running generate_beam_viz.py. It's not clear to me where these errors originate and whether they are the result of incorrectly-formatted data in the json file or a bug in generate_beam_viz.py.