Kernel specs REST API

The [REST API for kernel specs](https://github.com/ipython/ipython/wiki/IPEP-25:-Registry-of-installed-kernels#rest-api) is described in IPEP 25.
Thanks. Requiring auth for this is OK, though, I think - the main use of this will be for the icons of different kernels, and I assume the notebook view is only accessible to authenticated users.