Support string uidType

[derekwsgray]

PR that adds 'string' as an allowable relationship uidType.

Regarding: #83

[championswimmer]

awesome!!

[championswimmer]

you can ignore the travis failure, i'll update the dependencies

[codecov-io]

Codecov Report

Merging #84 into master will increase coverage by <.01%.
The diff coverage is 70%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   94.94%   94.95%   +<.01%     
==========================================
  Files          64       64              
  Lines        1782     1784       +2     
==========================================
+ Hits         1692     1694       +2     
  Misses         90       90

Impacted Files	Coverage Δ
lib/responseHelper.js	97.56% <ø> (ø)	⬆️
lib/postProcess.js	96.87% <100%> (ø)	⬆️
lib/routes/related.js	100% <100%> (ø)	⬆️
lib/routes/addRelation.js	100% <100%> (ø)	⬆️
lib/ourJoi.js	87.5% <40%> (+0.22%)	⬆️

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