Merge internal changes

Changelog:
- `f472d141`: Support tied embeddings in LSTM encoder/decoder
- `89e19d42`: Don't print alignment by default (use `--print-alignment` to re-enable it)
- `d2e2a1d4`: Add Transformer-based language model
- `c2794070`: Add new Transformer configuration for IWSLT
- `2fbfda0d`: Misc changes for pytorch-translate
- Miscellaneous bug fixes