Drop log4j2 by default

[dixel]

Switch to using slf4j on the library level and an attempt to use timbre/timbre-slf4j on the template level. This way the responsibility for handling logging now goes to the developers and interop between different logging backends becomes a bit easier.

[codecov-io]

Codecov Report

Merging #120 into master will increase coverage by 0.84%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #120      +/-   ##
==========================================
+ Coverage   82.78%   83.62%   +0.84%     
==========================================
  Files          11       11              
  Lines         482      464      -18     
  Branches       35       29       -6     
==========================================
- Hits          399      388      -11     
+ Misses         48       47       -1     
+ Partials       35       29       -6

Impacted Files	Coverage Δ
src/org/zalando/stups/friboo/system.clj	60% <ø> (-16.93%)	:x:
src/org/zalando/stups/friboo/dev.clj	100% <ø> (+7.69%)	:white_check_mark:
src/org/zalando/stups/friboo/system/http.clj	90.55% <ø> (+0.78%)	:white_check_mark:
src/org/zalando/stups/friboo/system/db.clj	86.44% <ø> (+5.08%)	:white_check_mark:

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[dryewo]

👍

[dryewo]

👍

[dixel]

:+1: