Made run-example respect SPARK_JAVA_OPTS and SPARK_MEM.

bin/run-example scripts was not passing Java properties set through the SPARK_JAVA_OPTS to the example. This is important for examples like Twitter** as the Twitter authentication information must be set through java properties. Hence added the same JAVA_OPTS code in run-example as it is in bin/spark-class script. 

Also added SPARK_MEM, in case someone wants to run the example with different amounts of memory. This can be removed if it is not tune with the intended semantics of the run-example scripts.

@matei Please check this soon I want this to go in 0.9-rc4