Support create/ensure for channels and actors

**NOTE**: This breaks the old `wes_channel` API

* Adds a new public API to the `wes` module
* Support both create and ensure for channels and actors
  (create produces an error when a channel/actor already exists, ensure
   just makes sure it exists).
* Channels and actors are now two-tuples
* Actor specifications now must have a start type (`create` or `load`)
* Added types and dialyzer fixes