Create REST API for kernel specs
Test kernel specs REST API

And fix kernel not found producing a 404 code.
Refactor kernelspec resource handler to separate URL prefix
Put full kernel info in REST API response for all kernels
Kernel resource handlers now require authenticated user
Use stdlib json, not zmq jsonapi
Merge pull request #5827 from takluyver/kernelspec-rest-api

Kernel specs REST API
Print sources for property fget/fset/fdel methods

This commit incorporates a backward-incompatible change:
`core.oinspect.getsource` will no longer accept `is_binary` kwarg.
core.interactiveshell.ofind: don't evaluate property.fget (it may raise)
Use inspect.getdoc when populating _builtin_type_docstrings

IPython.core.oinspect.getdoc uses inspect.getdoc which
strips some whitespace internally and because of that

    inspect.getdoc(property) != property.__doc__