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.
Renamed setCurrentJobDescription to setJobDescription.
Merge pull request #831 from rxin/scheduler

A few small scheduler / job description changes.