Merge pull request #1187 from minrk/nbshutdown Notebook cleanups and fixes: connection file cleanup, first heartbeat, startup flush. Kernels would not linger, but the KernelManagers are not garbage-collected on shutdown. This means that connection files for kernels still running at notebook shutdown would not be removed. Now, kernels are explicitly killed at server shutdown, allowing the KernelManagers to cleanup files. Small changes along the way: * disables the unnecessary (and actively detrimental) SIGINT handler inherited from the original copy/paste from the qt app. * put webapp initialization in `init_webapp` out of `initialize`, to preserve convention of there being no unique code in `initialize()`. * don't warn about http on all interfaces if running in 100% read-only mode, because no login or execution is possible. Closes #1232.