[BUILD] Runner for spark packages

This is a convenience method added to the SBT build for developers, though if people think its useful we could consider adding a official script that runs using the assembly instead of compiling on demand.  It simply compiles spark (without requiring an assembly), and invokes Spark Submit to download / run the package.

Example Usage:
```
$ build/sbt
> sparkPackage com.databricks:spark-sql-perf_2.10:0.2.4 com.databricks.spark.sql.perf.RunBenchmark --help
```

Author: Michael Armbrust <michael@databricks.com>

Closes #10834 from marmbrus/sparkPackageRunner.