Document libgfortran dependency for MLBase

Get rid of / improve ugly NPE when Utils.deleteRecursively() fails

Fix copy issue in https://github.com/mesos/spark/pull/899

Add better docs for coalesce.

listFiles() could return null if the I/O fails, and this currently results in an ugly NPE which is hard to diagnose.

Add metrics-ganglia to core pom file

Adding Manen dependency for Ganglia

Fix hash bug

Fix an instance where full standalone mode executor IDs were passed to

Small tweaks to MLlib docs

Job scheduling and cluster mode docs

updated content

Clean-up of Metrics Code/Docs and Add Ganglia Sink

StandaloneSchedulerBackend instead of the smaller IDs used within Spark
(that lack the application name).

This was reported by ClearStory in
https://github.com/clearstorydata/spark/pull/9.

Also fixed some messages that said slave instead of executor.

Fix target JVM version in scala build

Include the useful tip that if shuffle=true, coalesce can actually
increase the number of partitions.

This makes coalesce more like a generic `RDD.repartition` operation.

(Ideally this `RDD.repartition` could automatically choose either a coalesce or
a shuffle if numPartitions was either less than or greater than, respectively,
the current number of partitions.)

SPARK-660: Add StorageLevel support in Python