[SPARK-7201] [MLLIB] move Identifiable to ml.util

It shouldn't live directly under `spark.ml`.

Author: Xiangrui Meng <meng@databricks.com>

Closes #5749 from mengxr/SPARK-7201 and squashes the following commits:

53847f9 [Xiangrui Meng] move Identifiable to ml.util

mllib/src/main/scala/org/apache/spark/ml/Evaluator.scala

@@ -19,6 +19,7 @@ package org.apache.spark.ml
 
 import org.apache.spark.annotation.AlphaComponent
 import org.apache.spark.ml.param.ParamMap
+import org.apache.spark.ml.util.Identifiable
 import org.apache.spark.sql.DataFrame
 
 /**

mllib/src/main/scala/org/apache/spark/ml/param/params.scala

@@ -24,7 +24,7 @@ import scala.annotation.varargs
 import scala.collection.mutable
 
 import org.apache.spark.annotation.{AlphaComponent, DeveloperApi}
-import org.apache.spark.ml.Identifiable
+import org.apache.spark.ml.util.Identifiable
 
 /**
  * :: AlphaComponent ::

mllib/src/main/scala/org/apache/spark/ml/Identifiable.scala → mllib/src/main/scala/org/apache/spark/ml/util/Identifiable.scala

@@ -15,7 +15,7 @@
  * limitations under the License.
  */
 
-package org.apache.spark.ml
+package org.apache.spark.ml.util
 
 import java.util.UUID