Major Description
As described in the design doc we need to introduce a function to install Spark in case the user directly downloads SparkR from CRAN.
To do that we can introduce a install_spark function that takes in the following arguments
hadoop_version url_to_use # defaults to apache local_dir # defaults to a cache dir
Further more I think we can automatically run this from sparkR.init if we find Spark home and the JARs missing.
