Add support for passing Spark environment vars
This change creates a new `createSparkContext` method in RRDD as we can't pass
Map<String, String> through rJava. Also use SPARK_MEM in local mode to increase
heap size and update the README with some examples.
Use `$` for calling `put` instead of .jrcall
Merge pull request #22 from shivaram/master

Add support for passing Spark environment vars