upate exmaple notebooks to nbformat v4
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
Merge pull request #6885 from mattvonrocketstein/issue6883

Issue6883: nix traceback for shell.system if exception is KeyboardInterrupt
Remove separate 'path', 'name' in Contents API

- path is full path (including name)
- name is only used for read-only convenience, and plays no role in any API
- remove 'untitled upload' (POST with content and no path)
remove copy via PUT

only copy via POST to directories
remove notebook['name'] from sessions
update frontend with path/name changes

- remove name from models
- path is full path (contains name)
rename ContentsManager.create_file->new

consistent with js API
unicode!