update hmac signature comparison

see https://docs.python.org/2/library/hmac.html#hmac.compare_digest
support Python older than 2.7.7
minor cleanup
warn users running older versions of Python
return global events from require(['base/js/events']).

In master, it is impossible to load code on IPython events without setting timeouts.
don’t use events !== undefined to signal page ready

it will always be defined
Trigger app_initialized event *after* the 'global' IPython object is initialized
preserve backward-compatible $([IPython.events])
don’t load custom.js in page template

we are loading it in require, and don’t want to load it twice
fix docstring in events.js

example shows registering a handler, not triggering an event.
note loaded, unused dependencies in require
use global events in js tests
don’t store minified css in development

- setup css defaults to non-minified css
- build minified css and jsversion on sdist/wheel
remove css sourcemaps from repo
attempt to regen source maps in git-hooks

if css changes
regen css (non-minified)

via `setup.py css`
send ping every 30 seconds to keep websockets alive
add strict arg to css_js setup decorator

and attempt to build css on build_by (install)
`fab css` checks whether it needs to do anything

makefile-style

adds `force` arg to override, minify implies force.
check_call with list, instead of shell=True