Remove slf4j-simple from dependencies

[pcimcioch]

slf4j-simple is logging implementation and it shouldn't be forced as library's transient, non-optional dependency.
Adding slf4j-simple was quite disruptive, as it basically breaks applications that include this library but use other slf4j implementations (like logback).
From what I can see, it should be enough to have commons-logging in dependencies, and slf4j-simple can be used for test dependencies.

[codecov-commenter]

Codecov Report

Merging #14 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #14   +/-   ##
=========================================
  Coverage     72.81%   72.81%           
  Complexity      116      116           
=========================================
  Files            20       20           
  Lines           515      515           
  Branches         38       38           
=========================================
  Hits            375      375           
  Misses          118      118           
  Partials         22       22           

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[skjolber]

@robertotru any progress on this?