load and save functionality for open files and strings

[allisonportis]

fixes #73

Updated /featuretools/feature_base save and load to fulfill:

Returns the serialized features as a string
ft.save_features(features)

Saves the serialized features as a file
ft.save_features(features, '/path/to/save/to.txt')

Save the features to open file f
ft.save_features(features, f)

Loads features from string:
ft.load_features(feature_str)

Loads features from file path:
ft.load_features('/path/to/features')

Loads features from open file f:
ft.load_features(f)

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #566 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
+ Coverage   96.43%   96.47%   +0.03%     
==========================================
  Files         117      117              
  Lines        9505     9524      +19     
==========================================
+ Hits         9166     9188      +22     
+ Misses        339      336       -3

Impacted Files	Coverage Δ
featuretools/feature_base/features_serializer.py	100% <100%> (ø)	⬆️
featuretools/feature_base/features_deserializer.py	98.24% <100%> (+0.16%)	⬆️
...ests/primitive_tests/test_feature_serialization.py	100% <100%> (ø)	⬆️
...etools/tests/entityset_tests/test_serialization.py	100% <0%> (+3.75%)	⬆️

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[rwedge]

Looks good