exposing convert_xgboost API method (#318)

Author: scnakandala

onnxmltools/__init__.py

@@ -22,6 +22,7 @@
 from .convert import convert_lightgbm
 from .convert import convert_sklearn
 from .convert import convert_sparkml
+from .convert import convert_xgboost
 
 from .utils import load_model
 from .utils import save_model