Add denoising dataset for denoising autoencoder (#306)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/306

This uses a source dataset to generate a batch of {source: noisy source, target: original clean source} which allows us to train a denoising autoencoding component as part of a seq2seq model.

Reviewed By: xianxl

Differential Revision: D10078981

fbshipit-source-id: 026225984d4a97062ac05dc3a36e79b5c841fe9c