Allow users to set where SparkR is installed
This also adds a warning if somebody tries to call sparkR.init multiple times.
Add keys() and values() for the RDD class.
Merge pull request #104 from sun-rui/add_keys_values

Add keys() and values() for the RDD class.
Merge pull request #102 from hlin09/hlin09

Adds function flatMapValues.
Merge pull request #103 from shivaram/rlibdir-fix

[SPARKR-115] Allow users to set where SparkR is installed
Adds countByValue().
Adds function countByKey().