Added logging for serialization / deserialization.

Serialization when data is read/written using Hadoop is not
captured. The serialization logging relies on sampling; the
configuration parameter
spark.metrics.serializeTime.sampleProbability controls
the frequency of sampling.

Conflicts:
	core/src/main/scala/org/apache/spark/executor/TaskMetrics.scala
	core/src/main/scala/org/apache/spark/storage/BlockManager.scala