add kernelspec for IPython 3.0

Still WIP, 

But once https://github.com/ipython/ipython/pull/6126 is merge this will allow to change running kernel in a single notebook server. This add the minimal changes necessary to register a new kernel with IPython.

The things that are still to figure out are the modification Julia bring to tooltip, and the logo/favicon that does not change (yet).

![capture decran 2014-07-26 a 22 43 23](https://cloud.githubusercontent.com/assets/335567/3712236/8a881e10-1505-11e4-8ecb-88739b503de0.png)

[edit] kernel selector is now in the `Kernel > change kernel` Menu. 

and as you point out, overwrite is by default to true, so I don't need to set it later in copying the logo, but whatever.