fix ipython usage text

It does not describe current behavior, which is now like regular python.

The `[-c cmd | -m mod | file]` is taken directly from Python's own help output.
noted sub_channel being renamed to iopub_channel
Merge pull request #3079 from minrk/kernel_class

Actually use IPKernelApp.kernel_class and make it configurable.

This makes it easier to write IPKernel subclasses that implement custom logic, such as a kernel in a different language.  Tested in the creation of a native Julia kernel.
Merge pull request #3080 from ivanov/doc_iopub

noted sub_channel being renamed to iopub_channel
fix DisplayMagics.html docstring
Merge pull request #3083 from kmike/patch-1

fix DisplayMagics docstrings

relics from @minrk's copy/paste
mention `--` in usage
Merge pull request #3073 from minrk/helpstring

fix ipython usage text
Starting to refactor heart beating of notebook kernels.
More work on the heartbeat.
Made is_alive a method of KernelManager and MultiKernelManager.

It was a property, but it is better suited as a method now.
Added tests in both the KM and MKM case.
Removing last remnant of heartbeat from IOPubHandler.
Removing heartbeat config from the MappingKernelManager.
Adding KerelRestarter.
First go at adding autorestart to MultiKernelManager.
More work on autorestarting.
Adding absolute import to kernelrestarter.
Refactoring kernel restarting.
relocate redundantly-named kernel files

uses subpackages for different subclasses (blocking, ioloop)
update imports with new layout