Added a few callbacks

[smiLLe]

I was missing a few callbacks i found useful in flutter_redux. So here are:

• onInit
• onFirstBuild
• onDidChange
• onDispose

Implemented for StoreConnector and StoreConnection. I also added tests, however i am no expert in flutter tests :)

[codecov-io]

Codecov Report

Merging #38 into master will increase coverage by 5.35%. The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   85.71%   91.07%   +5.35%     
==========================================
  Files           1        1              
  Lines          35       56      +21     
==========================================
+ Hits           30       51      +21     
  Misses          5        5

Impacted Files	Coverage Δ
lib/flutter_built_redux.dart	91.07% <92.85%> (+5.35%)	:arrow_up:

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