Add first version of Vega ROC plots

explore/visualization/README.md

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+# A directory for creating Vega based visualizations
+
+In order to run these sample files, you should first start up a simple HTTP server such as:
+
+```sh
+python -m http.server 8000
+```
+
+Then navigate to that instance (http://localhost:8000/) and click on the file that you wish to view.

explore/visualization/data/sample_auc.csv

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+partition,feature_set,auc,legend_index
+train,full features,0.76,0
+test,full features,0.74,1
+train,covariates only,0.72,2
+test,covariates only,0.70,3