WIP: Support Filters Being Bound to Different Stages of Processing

Makes it possible for a filter definition to include a new stage key that can be set to before_validation, after_validation, request_wire, or response_wire; which binds the filter to fire off either before validation (the default), after validation, before being written to the wire in a request, or after being read from the wire from a response, respectively.

Still needs tests.

Depends on #167, closes #169, and closes #134.