Add /_membership support

[flimzy]

No description provided.

[codecov]

Codecov Report

Merging #240 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
+ Coverage   89.60%   89.62%   +0.01%     
==========================================
  Files          31       31              
  Lines        2223     2227       +4     
==========================================
+ Hits         1992     1996       +4     
  Misses        167      167              
  Partials       64       64              

Impacted Files	Coverage Δ
auth.go	100.00% <ø> (ø)
couchdb.go	100.00% <ø> (ø)
db.go	81.72% <ø> (ø)
cluster.go	100.00% <100.00%> (ø)

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