Set up CircleCI testing.

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[codecov-io]

Codecov Report

Merging #45 into master will increase coverage by 0.74%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   78.79%   79.54%   +0.74%     
==========================================
  Files          21       21              
  Lines        4306     4366      +60     
==========================================
+ Hits         3393     3473      +80     
+ Misses        913      893      -20

Impacted Files	Coverage Δ
nodepy/low_storage_rk.py	50.13% <0%> (+0.27%)	⬆️
nodepy/rooted_trees.py	91.5% <0%> (+6.03%)	⬆️

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[coveralls]

Coverage remained the same at 78.797% when pulling febc778 on circleci into 9954134 on master.