Documentation clarifications. Remove unused pykafka.common.Message.

[DieterDePaepe]

#917

[codecov-io]

Codecov Report

Merging #922 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #922      +/-   ##
==========================================
+ Coverage   83.15%   83.32%   +0.17%     
==========================================
  Files          36       36              
  Lines        3799     3797       -2     
  Branches      562      562              
==========================================
+ Hits         3159     3164       +5     
+ Misses        493      487       -6     
+ Partials      147      146       -1

Impacted Files	Coverage Δ
pykafka/topic.py	80.41% <ø> (-2.07%)	⬇️
pykafka/simpleconsumer.py	85.49% <ø> (-0.67%)	⬇️
pykafka/protocol/message.py	88.14% <100%> (ø)	⬆️
pykafka/common.py	100% <100%> (ø)	⬆️
pykafka/balancedconsumer.py	89.76% <0%> (-1.17%)	⬇️
pykafka/connection.py	83.16% <0%> (-1%)	⬇️
pykafka/handlers.py	91.91% <0%> (-0.74%)	⬇️
pykafka/cluster.py	72.52% <0%> (+0.36%)	⬆️
... and 3 more

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[emmettbutler]

Looks great, thanks for the pull request. There are still a few instances of common.Message in docstrings, including in the protocol and in simpleconsumer.py. Once those are adjusted, this can be merged.