Merge pull request #54: Version 0.1.0 -> 0.2.0

Release notes:
- 5c7f4954: Added simple LSTM model with input feeding and attention
- 6e4b7e22: Refactored model definitions and incremental generation to be cleaner
- 7ae79c12: Split interactive generation out of generate.py and into a new binary: interactive.py
- 19a3865d: Subtle correctness fix in beam search decoder. Previously, for a beam size of k, we might emit a hypotheses
           if the <eos> was among the top 2*k candidates. Now we only emit hypotheses for which the <eos> is among the
           top-k candidates. This may subtly change generation results, and in the case of k=1 we will now produce
           strictly greedy outputs.
- 97d7fcb9: Fixed bug in padding direction, where previously we right-padded the source and left-padded the target. We
           now left-pad the source and right-pad the target. This should not effect existing trained models, but may
           change (usually improves) the quality of new models.
- f442f896: Add support for batching based on the number of sentences (`--max-sentences`) in addition to the number of
           tokens (`--max-tokens`). When batching by the number of sentences, one can optionally normalize the gradients
           by the number of sentences with `--sentence-avg` (the default is to normalize by the number of tokens).
- c6d6256b: Add `--log-format` option and JSON logger