Add exclusions and explicit dependencies so that the examples
assembly does not duplicate classes already packaged in the main
assembly.

Also avoid relocating the commons-math3 package since it's already
a dependency of spark-core, and thus is already available in the
main assembly.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5379 from vanzin/examples-deps and squashes the following commits:

12c258e [Marcelo Vanzin] [minor] [examples] Avoid re-packaging unneeded classes.