Use enums for model_type instead of magic string.

[angela97lin]

Addresses #50 by using enums for model_type instead of magic strings.

[codecov]

Codecov Report

Merging #110 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   92.67%   92.74%   +0.06%     
==========================================
  Files          47       49       +2     
  Lines        1256     1268      +12     
==========================================
+ Hits         1164     1176      +12     
  Misses         92       92

Impacted Files	Coverage Δ
evalml/tests/pipeline_tests/test_pipelines.py	93.61% <100%> (ø)	⬆️
evalml/models/auto_base.py	91.5% <100%> (ø)	⬆️
evalml/model_types/__init__.py	100% <100%> (ø)
evalml/pipelines/classification/random_forest.py	80% <100%> (+0.68%)	⬆️
evalml/pipelines/regression/random_forest.py	100% <100%> (ø)	⬆️
evalml/tests/automl_tests/test_autoclassifier.py	100% <100%> (ø)	⬆️
...ml/pipelines/classification/logistic_regression.py	93.54% <100%> (+0.21%)	⬆️
evalml/model_types/model_types.py	100% <100%> (ø)
evalml/pipelines/classification/xgboost.py	100% <100%> (ø)	⬆️
evalml/__init__.py	100% <100%> (ø)	⬆️
... and 2 more

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[rwedge]

Looks good!