Add spark-tools assembly to run-example's classpath

This commit adds an assembly for `spark-tools` and adds it to `run-example`'s classpath, allowing the JavaAPICompletenessChecker to be run against Spark 0.8+ with

    ./run-example org.apache.spark.tools.JavaAPICompletenessChecker

Previously, this tool was run through the `run` script.  I chose to add this to `run-example` because I didn't want to duplicate code in a `run-tool` script.