Add user relation

[sgaynetdinov]

close #22

[codecov-io]

Codecov Report

Merging #23 into master will increase coverage by 0.93%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
+ Coverage   43.05%   43.98%   +0.93%     
==========================================
  Files          17       17              
  Lines         720      732      +12     
==========================================
+ Hits          310      322      +12     
  Misses        410      410

Impacted Files	Coverage Δ
vk/users.py	52.71% <100%> (+2.49%)	⬆️

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