[MRG] add masked_batch_dot layer, and its tests and docs

[nelson-liu]

Added a layer for doing masked_batch_dot.

Built Docs:

• API Summary page for layers: https://132-47793051-gh.circle-artifacts.com/0/home/ubuntu/deep_qa/doc/_build/html/layers/backend.html#maskedbatchdot
• Full API Reference page for layers: https://132-47793051-gh.circle-artifacts.com/0/home/ubuntu/deep_qa/doc/_build/html/api_doc/deep_qa.layers.backend.html#module-deep_qa.layers.backend.masked_batch_dot

[nelson-liu]

I don't think this is really needed anymore, as it's mostly superseded by #191. closing, let me know if you want to add it.