Add ZipCode Variable Type

[gsheni]

• This adds a zipcode variable type
• Fixed some import statement order and spacing

[codecov]

Codecov Report

Merging #367 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #367      +/-   ##
==========================================
+ Coverage   95.75%   95.79%   +0.04%     
==========================================
  Files          89       89              
  Lines        7751     7754       +3     
==========================================
+ Hits         7422     7428       +6     
+ Misses        329      326       -3

Impacted Files	Coverage Δ
featuretools/computational_backends/utils.py	94.9% <ø> (ø)	⬆️
featuretools/tests/dfs_tests/test_dfs_method.py	98.41% <ø> (ø)	⬆️
featuretools/tests/testing_utils/mock_ds.py	87.5% <100%> (+0.09%)	⬆️
featuretools/variable_types/variable.py	97% <100%> (+0.06%)	⬆️
featuretools/entityset/entity.py	95.65% <0%> (ø)	⬆️
...aturetools/primitives/standard/binary_transform.py	98.89% <0%> (+1.1%)	⬆️

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[kmax12]

@gsheni can you add this new variable type to the api reference? after that, good to merge

[gsheni]

@kmax12 Added to the api_reference.rst
Also, added zipcode column to mock_ds

[kmax12]

Looks good to merge