SPARK-4297 [BUILD] Build warning fixes omnibus

There are a number of warnings generated in a normal, successful build right now. They're mostly Java unchecked cast warnings, which can be suppressed. But there's a grab bag of other Scala language warnings and so on that can all be easily fixed. The forthcoming PR fixes about 90% of the build warnings I see now.
Maybe the SBT build doesn't run what the `antrun` plugin does in the Maven build? Here's the mojo doc that shows the deprecation: http://maven.apache.org/plugins/maven-antrun-plugin/run-mojo.html We should be using the same plugin since the Maven build defines the version.

Hm I wonder if this is a different little problem. Does the SBT build trigger ...
`<unzip src="../python/lib/py4j-0.8.2.1-src.zip" dest="../python/build" />`
Since it's python stuff I'd assume it doesn't directly impact the SBT build but would think it's necessary for tests?