ENH: use pytest instead of numpy for tests

[ankostis]

numpy was used for TCs just for its assert_raises method.

• Added test extras, like when pip install -e .[test]
• Use pytest & coverage on travis.
• As usual, contains FIX(build): py2 needs pinning networkx-2.2 #16 networkx fix to pass travis.
• Task of RFE: enhancements for the future of the project #22.

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[codecov-io]

Codecov Report

Merging #28 into master will decrease coverage by 0.28%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
- Coverage   77.87%   77.58%   -0.29%     
==========================================
  Files           5        5              
  Lines         348      348              
==========================================
- Hits          271      270       -1     
- Misses         77       78       +1

Impacted Files	Coverage Δ
graphkit/base.py	77.77% <0%> (-1.59%)	⬇️

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