Pipelayer

* Adds pipline event `step_end`

* Pipeline and Switch raise start/exit/end events
* Removes Filter pre_process/post_process methods in lieu of events to simplfy pipeline runner
* Condenses Manifest models

* Enables addition assigment operator for appending event handlers
* Adds `default` case for Switch filter
* BUGFIX: Patches typing imports for Python 3.7

* Adds start/exit/filter events to `pipelayer.Filter`
* Terminates pipeline if the Filter.exit event is raised
* Adds raise_event decorator to wrap the `run` method in an implementation of `pipeline.Filter`
* Consolidates create methods in `pipelayer.ManifestHelper`

* BUGFIX: Fixes check for staticmethods when initializing the pipeline.

* 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)