@pwendell 

I think we were already alphabetizing within each "block" of imports, so are we extending this to a total alphabetical ordering of all imports?

Should `com.google.common.base.Optional` appear at the top of the imports list?  Or should the `com.google.*` imports come before the `spark.*` imports in a separate section?

Can we document the new import style in https://github.com/mesos/spark/blob/master/docs/contributing-to-spark.md?