Updated for Hypertable version 0.9.7.12; Performance improvements

Hi Brian,

I'm the maintainer of Hypertable.  I've updated the Hypertable YCSB driver for the latest stable version of Hypertable (0.9.7.12).  I also made a number of performance improvements to the driver and updated the README.  Can you please pull in this commit?  Thank you!

- Doug