extended IntegerValue to only allow positive integers

[MikeWooster]

I have recently identified a requirement when working with django-configurations where I needed only a positive integer value. I thought others may benefit from this feature too.

[codecov]

Codecov Report

Merging #186 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   88.18%   88.33%   +0.14%     
==========================================
  Files          23       23              
  Lines        1143     1157      +14     
  Branches      104      105       +1     
==========================================
+ Hits         1008     1022      +14     
  Misses        106      106              
  Partials       29       29

Flag	Coverage Δ
#dj110	87.72% <100%> (+0.15%)	⬆️
#dj111	87.98% <100%> (+0.14%)	⬆️
#dj18	88.07% <100%> (+0.14%)	⬆️
#dj20	86.77% <100%> (+0.16%)	⬆️
#djmaster	86.77% <100%> (+0.16%)	⬆️
#py27	87.64% <100%> (+0.15%)	⬆️
#py34	87.12% <100%> (+0.15%)	⬆️
#py35	87.12% <100%> (+0.15%)	⬆️
#py36	87.12% <100%> (+0.15%)	⬆️
#pypy	87.64% <100%> (+0.15%)	⬆️

Impacted Files	Coverage Δ
configurations/values.py	95.11% <100%> (+0.09%)	⬆️
tests/test_values.py	100% <100%> (ø)	⬆️

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[blueyed]

Thanks!