Add basic implementation of the customer endpoint

[codecov[bot]]

Codecov Report

Merging #52 into master will increase coverage by 0.32%. The diff coverage is 95.4%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   74.18%   74.51%   +0.32%     
==========================================
  Files         149      152       +3     
  Lines       21757    21844      +87     
==========================================
+ Hits        16140    16276     +136     
+ Misses       5617     5568      -49

Impacted Files	Coverage Δ
src/commercetools/testing/__init__.py	89.77% <100%> (+0.23%)	:arrow_up:
src/commercetools/testing/customers.py	100% <100%> (ø)
tests/test_service_customers.py	100% <100%> (ø)
src/commercetools/client.py	95.51% <100%> (+0.05%)	:arrow_up:
src/commercetools/services/customers.py	87.09% <87.09%> (ø)
src/commercetools/schemas/_customer.py	81.08% <0%> (+0.9%)	:arrow_up:
src/commercetools/types/_customer.py	67.62% <0%> (+11.99%)	:arrow_up:

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[davidweterings]

LGTM 👍