Introduce StormTopologyBuilder

[ttim]

This PR introduces StormTopologyBuilder class and move all Topology creation code from Storm platform to it, to make it easier to read.

Apart from that this PR doesn't contain any changes in logic.

[codecov-io]

Codecov Report

Merging #730 into develop will increase coverage by 0.06%. The diff coverage is 92.38%.

@@             Coverage Diff             @@
##           develop     #730      +/-   ##
===========================================
+ Coverage    71.06%   71.12%   +0.06%     
===========================================
  Files          145      146       +1     
  Lines         3560     3564       +4     
  Branches       197      196       -1     
===========================================
+ Hits          2530     2535       +5     
+ Misses        1030     1029       -1

Impacted Files	Coverage Δ
.../com/twitter/summingbird/storm/StormPlatform.scala	64.28% <100%> (-18.63%)	:arrow_down:
...witter/summingbird/storm/FlatMapBoltProvider.scala	98.55% <100%> (ø)	:arrow_up:
...itter/summingbird/storm/StormTopologyBuilder.scala	100% <100%> (ø)
...la/com/twitter/summingbird/storm/BuildSummer.scala	43.47% <66.66%> (+1.25%)	:arrow_up:
.../com/twitter/summingbird/storm/SpoutProvider.scala	90.19% <88.88%> (ø)	:arrow_up:
.../twitter/summingbird/memory/ConcurrentMemory.scala	95.71% <0%> (-1.43%)	:arrow_down:
...a/com/twitter/summingbird/storm/StormTestRun.scala	66.66% <0%> (+6.66%)	:arrow_up:

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[ttim]

@benpence yes, just to encapsulate topology build logic into one place.

[pankajroark]

lgtm