A few small scheduler / job description changes.

1. Renamed SparkContext.addLocalProperty to setLocalProperty. And allow this function to unset a property.

2. Renamed SparkContext.setDescription to setCurrentJobDescription.

3. Throw an exception if the fair scheduler allocation file is invalid.

Adding GC Stats to TaskMetrics (and three small fixes)

Fixed 2 bugs in executor UI (incl. SPARK-877)

1) UI crashed if the executor UI was loaded before any tasks started.
2) The total tasks was incorrectly reported due to using string (rather
than int) arithmetic.

Set SPARK_CLASSPATH for maven repl tests

Update to Mesos 0.12.1

This fixes an error where empty cells are missing if a given task
has no shuffle read/write.

Print run command to stderr rather than stdout

Properly account for killed tasks.

The TaskState class's isFinished() method didn't return true for
KILLED tasks, which means some resources are never reclaimed
for tasks that are killed. This also made it inconsistent with the
isFinished() method used by CoarseMesosSchedulerBackend.

Minor clean-up in metrics servlet code

Add MetricsServlet for Spark metrics system

Simple PageRank algorithm implementation in Python for SPARK-760

Implementing SPARK-865: Add the equivalent of ADD_JARS to PySpark

Now ADD_FILES uses a comma as file name separator.

Create SparkContext in beforeAll for MLLib tests

Change scala.Option to Guava Optional in Java APIs

Clean up scaladoc in ML Lib.

Changed yarn.version to 2.0.5 in pom.xml

Remove now dead code inside of docs

Also build and copy ML Lib scaladoc in Spark docs build.
Some more minor cleanup with respect to naming, test locations etc.