add opentracing support

[vetinari]

no initialization of opentracing, just adding decorators and adding tags / log_kv to spans

fixes #80

[codecov[bot]]

Codecov Report

Merging #81 into master will increase coverage by 1.88%. The diff coverage is 98.14%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   76.04%   77.92%   +1.88%     
==========================================
  Files          18       18              
  Lines        1056     1155      +99     
==========================================
+ Hits          803      900      +97     
- Misses        253      255       +2

Impacted Files	Coverage Δ
zmon_cli/client.py	97.92% <98.14%> (-0.06%)	:arrow_down:
zmon_cli/cmds/__init__.py	100% <0%> (ø)	:arrow_up:

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[vetinari]

:+1:

[vetinari]

:+1:

[mohabusama]

👍