Implement installation of wrapper kernels

[anntzer]

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 :-))