RUBBoS workload

Hello,

I've implemented the *Rice University Bulletin Board System* (RUBBoS) workload.
RUBBoS is a bulletin board benchmark modeled after an online news forum like [Slashdot](http://slashdot.org).
RUBBoS is a free, open source initiative available at the [OW2 Consortium](http://jmob.ow2.org/rubbos.html) web site.

I've implemented the workload driver for two possible incarnations of RUBBoS, namely `php` (target at the RUBBoS implementation based on PHP) and `servlet` (target at the RUBBoS implementation based on the Java Servlet technology).
You can select what version of RUBBoS to use by the `rubbos.incarnation` configuration parameter.

Also,
- I've tried to make the driver fully compliant with the native client `edu.rice.rubbos.client.ClientEmulator`,
- I've implemented all 24 operations + 2 special operations (i.e., "Back to Previous Page" and "End of Session"), provided by the original RUBBoS version as well,
- I've added three traffic matrices ("user-default", "author-default", "browse") as available in the original RUBBoS, and
- I've added several configuration options to provide more flexibility on the user side.

See:

https://github.com/sguazt/rain-workload-toolkit/tree/wkl-rubbos/src/radlab/rain/workload/rubbos/README.md

for a more detailed description of this workload.

Please, consider to merge this branch.

Best,

-- Marco