Optimize PySpark worker invocation

- Exclude a version of ASM 3.x that comes from HBase
- Don't use a special ASF repo for HBase
- Update SLF4J version
- Add sbt-dependency-graph plugin so we can easily find dependency trees

fix typo and coding style in #638

Conflicts:
	project/SparkBuild.scala

Add hBase example (retry of pull request #596)

Reduce ZippedPartitionsRDD's getPreferredLocations complexity from O(2^2n) to O(2^n)

Bug fix: Zero-length partitions result in NaN for overall mean & variance

SPARK-781: Log the temp directory path when Spark says "Failed to create temp directory."

Conflicts:
	core/src/main/scala/spark/scheduler/cluster/TaskSetManager.scala

temp directory".

Enhance pipe to support more features we can do in hadoop streaming

Add zeroLengthPartitions() test to make sure, e.g., StatCounter.scala can handle empty partitions without incorrectly returning NaN

Add top K method to RDD using a bounded priority queue

SPARK-772 / SPARK-774: groupByKey and cogroup should disable map side combine