[SPARK-4710] [mllib] Eliminate MLlib compilation warnings

Renamed StreamingKMeans to StreamingKMeansExample to avoid warning about name conflict with StreamingKMeans class.

Added import to DecisionTreeRunner to eliminate warning.

CC: mengxr

Author: Joseph K. Bradley <joseph@databricks.com>

Closes #3568 from jkbradley/ml-compilation-warnings and squashes the following commits:

64d6bc4 [Joseph K. Bradley] Updated DecisionTreeRunner.scala and StreamingKMeans.scala to eliminate compilation warnings, including renaming StreamingKMeans to StreamingKMeansExample.

(cherry picked from commit 4ac21511547dc6227d05bf61821cd2d9ab5ede74)
Signed-off-by: Xiangrui Meng <meng@databricks.com>