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
Deduplicate Local and Cluster schedulers.

The code in LocalScheduler/LocalTaskSetManager was nearly identical
to the code in ClusterScheduler/ClusterTaskSetManager. The redundancy
made making updating the schedulers unnecessarily painful and error-
prone. This commit combines the two into a single TaskScheduler/
TaskSetManager.
Fixed most issues with unit tests
Cleaned up imports and fixed test bug
Extracted TaskScheduler interface.

Also changed the default maximum number of task failures to be
0 when running in local mode.
Merge remote-tracking branch 'upstream/master' into consolidate_schedulers

Conflicts:
	core/src/main/scala/org/apache/spark/scheduler/ClusterScheduler.scala
Fixed naming issues and added back ability to specify max task failures.
Changed local backend to use Akka actor
Fix bug where scheduler could hang after task failure.

When a task fails, we need to call reviveOffers() so that the
task can be rescheduled on a different machine. In the current code,
the state in ClusterTaskSetManager indicating which tasks are
pending may be updated after revive offers is called (there's a
race condition here), so when revive offers is called, the task set
manager does not yet realize that there are failed tasks that need
to be relaunched.
Don't retry tasks if result wasn't serializable
Merge remote-tracking branch 'upstream/master' into consolidate_schedulers

Conflicts:
	core/src/main/scala/org/apache/spark/scheduler/cluster/ClusterTaskSetManager.scala
Fixed error message in ClusterScheduler to be consistent with the old LocalScheduler
SPARK-968, In stage UI, add an overview section that shows task stats grouped by executor id