Add badges to README

[thisac]

• Add badges for PyPI, Python, Codecov and CircleCI.
• Ignore coverage for cached_property. Already tested with Python 3.7 tests.

[codecov-commenter]

Codecov Report

Merging #10 (879a55f) into master (30fa5a8) will increase coverage by 2.74%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
+ Coverage   87.09%   89.84%   +2.74%     
==========================================
  Files          15       15              
  Lines        1178     1142      -36     
  Branches      185      179       -6     
==========================================
  Hits         1026     1026              
+ Misses        152      116      -36     

Impacted Files	Coverage Δ
dwave/gate/utils.py	100.00% <ø> (+90.00%)	:arrow_up:

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