Default URI for files in session requests should be fs.defaultFS

Currently, paths are just passed directly to Spark for processing; that means that by default they refer to local files. That doesn't make a lot of sense in Livy since "local" means the server's filesystem, which most clients won't even have access to.

Instead, paths without a proper scheme should inherit the fs.defaultFS configuration from Hadoop; that should also make the 0.2 session creation API more backwards compatible, since it was possible to set up a custom default FS in 0.1.