Add a maven build file for SparkR

SBT relies on downloading a bunch of artifacts from the internet and doesn't work well behind proxies / firewalls. This changes adds an option to build SparkR using maven instead. Most of the maven build file is based on Spark's pom.xml [0] and [1]. 

This will help address #5.

[0] https://github.com/apache/incubator-spark/blob/master/pom.xml
[1] https://github.com/apache/incubator-spark/blob/master/assembly/pom.xml