Implement string hash code in C.

[SPARKR-135](https://sparkr.atlassian.net/browse/SPARKR-135). Improve wordcount performance by implement the string hash code in C. The performance improvement is about 10X.
On a 3-worker-node Spark cluster, each node has 16 threads (2.93Ghz Xeon), 48G memory:

3.2 G data  from 96 minutes to 8 minutes
320 M data from 10 minutes to 1 minute

@shivaram I haven't test this patch on Windows, could you help to test it? Thanks.
It'll be good to add a comment here saying what the algorithm is doing (and external pointers if this is based on something etc.)