[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.
[SPARK-4708][MLLib] Make k-mean runs two/three times faster with dense/sparse sample

Note that the usage of `breezeSquaredDistance` in
`org.apache.spark.mllib.util.MLUtils.fastSquaredDistance`
is in the critical path, and `breezeSquaredDistance` is slow.
We should replace it with our own implementation.

Here is the benchmark against mnist8m dataset.

Before
DenseVector: 70.04secs
SparseVector: 59.05secs

With this PR
DenseVector: 30.58secs
SparseVector: 21.14secs

Author: DB Tsai <dbtsai@alpinenow.com>

Closes #3565 from dbtsai/kmean and squashes the following commits:

08bc068 [DB Tsai] restyle
de24662 [DB Tsai] address feedback
b185a77 [DB Tsai] cleanup
4554ddd [DB Tsai] first commit