Add better docs for coalesce.

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.)
Reword 'evenly distributed' to 'distributed with a hash partitioner.
Use a set since shuffle could change order.
Add metrics-ganglia to core pom file
Merge pull request #912 from tgravescs/ganglia-pom

Add metrics-ganglia to core pom file
Print out more friendly error if listFiles() fails

listFiles() could return null if the I/O fails, and this currently results in an ugly NPE which is hard to diagnose.
Merge pull request #907 from stephenh/document_coalesce_shuffle

Add better docs for coalesce.
Fix copy issue in https://github.com/mesos/spark/pull/899
Style fix: put body of if within curly braces
Merge pull request #916 from mateiz/mkdist-fix

Fix copy issue in https://github.com/mesos/spark/pull/899
Merge pull request #915 from ooyala/master

Get rid of / improve ugly NPE when Utils.deleteRecursively() fails
Document fortran dependency for MLBase
Merge pull request #917 from pwendell/master

Document libgfortran dependency for MLBase