Store datetime properties as Python datetime objects in models

[mkistler]

This PR adds helper functions to convert to/from iso8601 datetimes and uses these functions in the _from_dict() and _to_dict() methods of the models so that datetime values are stored as datetime objects and not simply strings.

The PR also includes a minor update to VisualRecognitionV3 resulting from a recent update to the swagger.

[codecov-io]

Codecov Report

Merging #303 into v1.0.0 will increase coverage by 0.8%.
The diff coverage is 35.95%.

@@            Coverage Diff            @@
##           v1.0.0     #303     +/-   ##
=========================================
+ Coverage   37.29%   38.09%   +0.8%     
=========================================
  Files          24       24             
  Lines        8173     8187     +14     
=========================================
+ Hits         3048     3119     +71     
+ Misses       5125     5068     -57

Impacted Files	Coverage Δ
watson_developer_cloud/watson_service.py	82.6% <100%> (+0.38%)	⬆️
watson_developer_cloud/visual_recognition_v3.py	28.92% <22.22%> (-0.13%)	⬇️
..._developer_cloud/natural_language_classifier_v1.py	35.29% <33.33%> (+0.34%)	⬆️
watson_developer_cloud/discovery_v1.py	29.1% <4.34%> (+0.03%)	⬆️
watson_developer_cloud/conversation_v1.py	39.92% <46.93%> (+2.82%)	⬆️

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