Implement installation of wrapper kernels

ipykernel could provide the necessary machinery for installation of any wrapper kernel, rather than having to implement it again and again (as in https://github.com/takluyver/bash_kernel/blob/master/bash_kernel/install.py). See e.g. https://github.com/Calysto/metakernel/pull/97/files#diff-791c3f643c36601ccb872bba3375d6d4R660 for a PR that uses `KernelClass.kernel_json` to create the relevant json file and let you use `python -mkernel install` to install it (dynamic creation of the json is helpful to avoid hardcoding the path to `python`, of course).
(Note that otherwise, all wrapper kernels still expose `python -mkernel install`, but that installs the ipykernel itself :-))