Add unit test code coverage generation to maven

We wanted unit test code coverage numbers for spark so I added Jacoco support to Maven.   I see there is a jacoco4sbt for sbt but I haven't had time to try it.  

Putting this up \for comments, perhaps better solution for scala code that works better across sbt and maven, or just preferred tool?  If people like this option I'll try to look at the jacoco4sbt when I have time (or perhaps there is someone more familiar with sbt that will volunteer).

This generates a jacoco-ut.exec that Jenkins can read when you run the tests and it also generates the coverage reports (html, xml, csv) if you run the package command.