Add gokv.Store implementation for MongoDB

[philippgille]

Closes #27

[codecov-io]

Codecov Report

Merging #30 into develop will increase coverage by 0.46%.
The diff coverage is 86.66%.

@@             Coverage Diff             @@
##           develop      #30      +/-   ##
===========================================
+ Coverage    82.02%   82.49%   +0.46%     
===========================================
  Files            8        9       +1     
  Lines          534      594      +60     
===========================================
+ Hits           438      490      +52     
- Misses          60       65       +5     
- Partials        36       39       +3

Impacted Files	Coverage Δ
mongodb/mongodb.go	86.66% <86.66%> (ø)

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