Added a serializable wrapper for HyperLogLog
Added a countDistinct method to RDD that takes takes an accuracy parameter and returns the (approximate) number of distinct elements in the RDD.
Added countDistinctByKey to PairRDDFunctions that counts the approximate number of unique values for each key in the RDD.
Added dependency on stream-lib version 2.4.0 for approximate distinct count support.
Fixed document typo
Improved code style.
Added stream-lib dependency to Maven build
Made SerializableHyperLogLog Externalizable and added Kryo tests
Removed superfluous abs call from test cases.
Fixed multiple file stream and checkpointing bugs.
- Made file stream more robust to transient failures.
- Changed Spark.setCheckpointDir API to not have the second
  'useExisting' parameter. Spark will always create a unique directory
  for checkpointing underneath the directory provide to the funtion.
- Fixed bug wrt local relative paths as checkpoint directory.
- Made DStream and RDD checkpointing use
  SparkContext.hadoopConfiguration, so that more HDFS compatible
  filesystems are supported for checkpointing.
Merge branch 'scheduler-update' into filestream-fix

Conflicts:
	core/src/main/scala/org/apache/spark/rdd/CheckpointRDD.scala
	streaming/src/main/scala/org/apache/spark/streaming/StreamingContext.scala
	streaming/src/main/scala/org/apache/spark/streaming/dstream/FileInputDStream.scala
	streaming/src/main/scala/org/apache/spark/streaming/scheduler/JobGenerator.scala
	streaming/src/test/scala/org/apache/spark/streaming/CheckpointSuite.scala
Merge branch 'scheduler-update' into filestream-fix
Updated CheckpointWriter and FileInputDStream to be robust against failed FileSystem objects. Refactored JobGenerator to use actor so that all updating of DStream's metadata is single threaded.
Fixed bug in file stream that prevented some files from being read
correctly.
Merge branch 'scheduler-update' into filestream-fix
Updated testsuites to work with the slack time of file stream.
Minor formatting fixes.
spark-544, introducing SparkConf and related configuration overhaul.
Merge branch 'apache-master' into filestream-fix
Fixed Python API for sc.setCheckpointDir. Also other fixes based on Reynold's comments on PR 289.