Dynamically add jar into Spark Driver's classpath.

Spark support dynamically add jar to executor classpath through SparkContext::addJar(), while it does not support dynamically add jar into driver classpath. In most case(if not all the case), user dynamically add jar with SparkContext::addJar() because some classes from the jar would be referred in upcoming Spark job, which means the classes need to be loaded in Spark driver side either,e.g during serialization. I think it make sense to add an API to add jar into driver classpath, or just make it available in SparkContext::addJar(). HIVE-9410 is a real case from Hive on Spark.