* Adds Switch filter
* Adds protocols:
* `pipelayer.protocol.CompoundStep`
* `pipelayer.protocol.Filter`
* `pipelayer.protocol.Manifest`
* Moves `Step` protocol to the `piplayer.protocol` namespace
* Moves `Pipeline` into separate module
* Exposes all core classes in root `__init__.py`
* Updates LICENSE to Free BSD (previous versions still honor the MIT License)