Add image transition when button opens/closes

[jjochen]

resolves #45

[codecov]

Codecov Report

Merging #46 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #46   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           6      6           
  Lines         596    655   +59     
  Branches       20     23    +3     
=====================================
+ Hits          596    655   +59

Impacted Files	Coverage Δ
...nButton/Classes/Extensions/UIView+Extensions.swift	100% <100%> (ø)	⬆️
...gActionButton/Classes/JJFloatingActionButton.swift	100% <100%> (ø)	⬆️

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