Check dtypes match when adding relationship

[rwedge]

Also try to convert categorical dtype to int dtype before converting it to object dtype

[kmax12]

Good to merge once tests pass

[codecov-io]

Codecov Report

Merging #159 into master will increase coverage by 0.39%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage   92.62%   93.02%   +0.39%     
==========================================
  Files          72       72              
  Lines        7729     7741      +12     
==========================================
+ Hits         7159     7201      +42     
+ Misses        570      540      -30

Impacted Files	Coverage Δ
featuretools/entityset/entity.py	91.76% <ø> (+0.96%)	⬆️
featuretools/entityset/base_entityset.py	91.3% <100%> (+0.54%)	⬆️
featuretools/tests/demo_tests/test_demo_data.py	100% <100%> (ø)	⬆️
featuretools/demo/mock_customer.py	94.73% <100%> (+73.68%)	⬆️
featuretools/tests/entityset_tests/test_es.py	98.98% <100%> (+0.08%)	⬆️
featuretools/entityset/entityset.py	90.75% <50%> (-0.66%)	⬇️
featuretools/synthesis/deep_feature_synthesis.py	92.87% <0%> (+0.27%)	⬆️

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