Fix PySpark unit tests on Python 2.6

Various UI improvements.

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