Add COO.nonzero method.

[hameerabbasi]

Closes #145

[codecov-io]

Codecov Report

Merging #148 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
+ Coverage   95.81%   95.82%   +<.01%     
==========================================
  Files          10       10              
  Lines        1195     1197       +2     
==========================================
+ Hits         1145     1147       +2     
  Misses         50       50

Impacted Files	Coverage Δ
sparse/coo/core.py	93.71% <100%> (+0.03%)	⬆️

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[mrocklin]

Nice to see this come out so easily