upate exmaple notebooks to nbformat v4
DEV: Support for configurable list of extra jinja template directories.
DEV: Add IPython.html to the default template path.

This makes it possible to override a file with a template that also
inherits from that file.

For example, this makes it possible to override a single block of
notebook.html by creating a file called notebook.html that inherits from
templates/notebook.html.
DOC: Fix typo in comment.
DOC: Add whatsnew entry for userspace templates.
Merge pull request #6861 from quantopian/template_dir_option

DEV: Support for configurable list of extra jinja template directories.
Fix typo in docstring.
Merge pull request #6880 from francisco-dlp/fix_typo

Fix typo in docstring.
Document limitations of symlink parameter to install_nbextension

Closes gh-6239
Merge pull request #6882 from takluyver/i6239-docstring

Document limitations of symlink parameter to install_nbextension
add exponential falloff for reconnect

avoids constant reconnect attempts every 3 seconds forever

gives up after 6 tries (last timeout 64s)
[issue547] add emacs tmp files and .coverage files to gitignore
add sticky `Connection lost` notification

when reconnect process gives up
add Reconnect to kernel menu
[issue6883] catch keyboardinterrupt if generated during shell.system() calls
add events to ws_closed_error in kernel.js
wait for kernel_ready after ws_closed_error
s/Connection lost/Not Connected/
Merge pull request #6843 from minrk/nbformat4-examples

update example notebooks to v4
Merge pull request #6876 from minrk/falloff-reconnect

Add exponential falloff to kernel reconnect