Fix handling of empty SPARK_EXAMPLES_JAR
Add warning to null setJars check
Change protobuf version for yarn alpha back to 2.4.1
Merge pull request #330 from tgravescs/fix_addjars_null_handling

Fix handling of empty SPARK_EXAMPLES_JAR

Currently if SPARK_EXAMPLES_JAR is left unset you get a null pointer exception when running the examples (atleast on spark on yarn).  The null now gets turned into a string of "null" when its put into the SparkConf so addJar no longer properly ignores it. This fixes that so that it can be left unset.
Merge pull request #342 from tgravescs/fix_maven_protobuf

Change protobuf version for yarn alpha back to 2.4.1

The maven build for yarn-alpha uses the wrong protobuf version and hence the generated assembly jar doesn't work with Hadoop 0.23.  Removing the setting for the yarn-alpha profile since the default protobuf version is 2.4.1 at the top of the pom file.