Add is_string utils function

[Seth-Rothschild]

This PR replaces the python2 style isinstance(value, basestring) with a utils function is_string(value). It replaces the import from past.builtins import basestring with our own function from featuretools.utils import is_string. The only other use of past.builtins is in serialization.py where we import unicode.

This is related to #255 and the associated issue #254.

[codecov-io]

Codecov Report

Merging #260 into master will decrease coverage by <.01%.
The diff coverage is 95.83%.

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
- Coverage   94.14%   94.13%   -0.01%     
==========================================
  Files          71       71              
  Lines        7632     7638       +6     
==========================================
+ Hits         7185     7190       +5     
- Misses        447      448       +1

Impacted Files	Coverage Δ
featuretools/synthesis/deep_feature_synthesis.py	93.27% <100%> (ø)	⬆️
featuretools/entityset/timedelta.py	80.71% <100%> (ø)	⬆️
featuretools/entityset/entity.py	89.77% <100%> (ø)	⬆️
featuretools/utils/api.py	100% <100%> (ø)	⬆️
featuretools/primitives/cum_transform_feature.py	97.63% <100%> (ø)	⬆️
featuretools/utils/gen_utils.py	68.62% <100%> (+1.96%)	⬆️
featuretools/utils/wrangle.py	67.88% <100%> (ø)	⬆️
featuretools/variable_types/variable.py	92.3% <100%> (ø)	⬆️
featuretools/primitives/utils.py	85.36% <66.66%> (ø)	⬆️

---

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[codecov-io]

Codecov Report

Merging #260 into master will increase coverage by <.01%.
The diff coverage is 95.83%.

@@            Coverage Diff             @@
##           master     #260      +/-   ##
==========================================
+ Coverage   94.14%   94.14%   +<.01%     
==========================================
  Files          71       71              
  Lines        7632     7638       +6     
==========================================
+ Hits         7185     7191       +6     
  Misses        447      447

Impacted Files	Coverage Δ
featuretools/synthesis/deep_feature_synthesis.py	93.27% <100%> (ø)	⬆️
featuretools/entityset/timedelta.py	80.71% <100%> (ø)	⬆️
featuretools/entityset/entity.py	89.77% <100%> (ø)	⬆️
featuretools/utils/api.py	100% <100%> (ø)	⬆️
featuretools/primitives/cum_transform_feature.py	97.63% <100%> (ø)	⬆️
featuretools/utils/gen_utils.py	70.58% <100%> (+3.92%)	⬆️
featuretools/utils/wrangle.py	67.88% <100%> (ø)	⬆️
featuretools/variable_types/variable.py	92.3% <100%> (ø)	⬆️
featuretools/primitives/utils.py	85.36% <66.66%> (ø)	⬆️

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 33c357b...afa5052. Read the comment docs.

[kmax12]

Looks good. merging