GitLab

When downloading files in examples directory (e.g. when running
`prepare-iwslt14.sh`), git sees them as untracked files but they should
not be committed.
Add a .gitignore script that ignore everything in the examples
subdirectories except for .sh and .md files.

A Task defines the data format, stores shared state (e.g., dictionaries) and provides helpers for building the model/criterion and calculating the loss.

Changes:
- Add TranslationTask and LanguageModelingTask. New tasks can be registered with @register_task decorator.
- Add EpochBatchIterator to encapsulate batching and saving/restoring dataloader position
- Remove LEFT_PAD_* constants and make them configurable per task