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.
Merge branch 'apache-master' into filestream-fix
Fixed Python API for sc.setCheckpointDir. Also other fixes based on Reynold's comments on PR 289.
Removed slack time in file stream and added better handling of exceptions due to failures due FileNotFound exceptions.
Changed file stream to not catch any exceptions related to finding new files (FileNotFound exception is still caught and ignored).
Added warning if filestream adds files with no data in them (file RDDs have 0 partitions).
Minor changes in comments and strings to address comments in PR 289.
Fixed comments and long lines based on comments on PR 289.
Bug fixes to the DriverRunner and minor changes here and there.
Merge branch 'filestream-fix' into driver-test

Conflicts:
	streaming/src/main/scala/org/apache/spark/streaming/Checkpoint.scala