remove cython extension.

Now in cython package itself, as stable.
remove Cython Built-in extension notebook
remove more ref to cythonmagic
use Cython.load_ipython_ext to simplify
add whatsnew and uppercase C in load_ext

Use uppercase as this is what is in cython docs.
Use contentEditable to allow modification via the the slider readout
Add support to the float slider
Add keydown listener to commit changes on <enter>
Ignore the event object for handleTextChange
Add support for parsing pairs of numbers for range sliders
Change _parse_text_input to _parse_value and update float range regex
Make comm manager (mostly) independent of InteractiveShell

This makes it possible to use comms from wrapper kernels, without
instantiating the full IPython shell machinery.

The one remaining place where we need a reference to shell is to fire
pre_execute and post_execute hooks (which are needed to get mpl figures
right). This is a pure IPythonism, that it should be safe to ignore if
shell is not set.
Make comm_manager a property of kernel, not shell
Require base/js/keyboard and use keycode.enter
Safeguard against comm instantiation with both session and kernel (corner case)
qtconsole: strip newline on text copy (fixes #6234)

IPython's inputsplitter inserts newline at the end of string passed to
it. So, if user didn't select newline before copying, we must strip it.
Fix code_to_line when called with empty code

Previously, it was referring to line before it was assigned. Noticed
when testing the R kernel.
trigger trust_changed properly on load

event signature was not consistent
Clean up session class
Fix reference to session in notebook.js