Add 'findExecutableOnSystemPath' method; eliminate Guava

In this release, I added a new findExecutableOnSystemPath function to the FileUtils class. This will eliminate reliance on similar Selenium APIs in Selenium Foundation.

I also removed dependencies on Guava APIs. These really weren't doing very much for me, and including this library could cause mixed-version issues for client projects.

NOTE: This is a breaking change, because the Params class has now switched from using the Guava Optional class to the standard Java Optional class.