Remove flush() and batch_len.

Going to take a different direction with batching. Also increment major
version because this is backwards incompatible.
Add _Pipeline, accessor, and docs.
Use __all__ instead of _ prefix.
Bump docs version.
Clear pipeline._stats after send().
Update CHANGES.
Flake8 cleanup.
Update requirements and test versions.

- Update nose and mock.
- Add flake8 to dev requirements.
- Add Py33 to tox and travis.