General cleanup of kernel manager code.

This does some general cleanup of MultiKernelManager to better reflect how it is actually being used.

Sometimes there is a need to create kernel_id's elsewhere in code.  This minor change allows a kernel_id to be created outside of the MultiKernelManager and passed in as a kwarg.  An exception is raised if the id is already used.

Tasks:

- [x] Fix cleanup of ipc files.
- [x] Allow kernel_id to be passed to MultiKernelManager.start_kernel.
- [x] Add ipc support to MultiKernelManager.
- [x] Add more tests for MultiKernelManager.
- [x] Rename sub channel to iopub channel everywhere.
- [x] Use consistent naming for all channel classes in zmq, inprocess and qt.
- [x] Move BlockingChannelMixin to zmq.blockingkernelmanager.
- [x] Create ABC for KernelManager.
- [x] Make the InProcessKernelManager a Configurable.
- [x] Cleanup docstrings in ABCs.
- [x] Add tests for KernelManager.
- [x] Check over MultiKernelManager.
- [x] Make KernelManager,kill_kernel private in ABC and implementations.
- [x] Find bug that is causing the kernel manager tests to hang unless the shell channel linger is set to 0.
- [x] Decide about critical logging in ipkernel.
- [x] Debug lack of stderr redirect in tests.