add ability to bind to a ruby kernel

This need the iruby kernel of my repo, (https://github.com/Carreau/iruby)
And to be modified as ruby interpreter and zmq-kernel path in IPython/zmq/rubykernel.py

Launch with (for example)
ipython notebook
--MappingKernelManager.kernel_manager_class='IPython.zmq.rubykernel.RubyKernelManager'

need zmq, uuid and hmac gem, and ruby 1.9.3

I would love to change the ruby interpreter path to `ruby1.9.3` but Even with latest ruby installed, it is not in my path, so I don't know how what the ruby way is.

Also any idea of how we want to managed external kernel ? Do we put them in main IPython repo ? keep them separate ?