kafka adapter example fails

[ngoldbaum]

Describe the bug Running e_08_kafka.py errors out

To Reproduce Steps to reproduce the behavior:

cd csp/examples
python e_08_kafka.py

Traceback (most recent call last):
  File "/Users/goldbaum/Documents/csp/csp/examples/e_08_kafka.py", line 6, in <module>
    from csp.adapters.kafka import (
ImportError: cannot import name 'ProtoMessageMapper' from 'csp.adapters.kafka' (/Users/goldbaum/Documents/csp/csp/adapters/kafka.py)

Expected behavior

No error

[NeejWeej]

I don't think ProtoMessageMapper is defined anywhere, though it is mentioned here: docs/wiki/api-references/Input-Output-Adapters-API.md