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 #16networkx fix to pass travis.
• Task of RFE: enhancements for the future of the project #22.

---

I confirm that this contribution is made under the terms of the license found in the root directory of this repository's source tree and that I have the authority necessary to make this contribution on behalf of its copyright owner.

[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%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e70718b...4e55b30. Read the comment docs.