add a notion of admin users

and an @admin_only decorator for restricted methods
simplify handler setup with default_handlers in modules

like IPython's, but a bit simpler since we don't have
so many services to deal with.
minor fixes to testing utils
add user list handler, first of many
add mock single-user server for testing
flesh out REST API

can now list/view/add/create/modify users
and start/stop single-user servers
update base page template

- no body params, just write jhapi globals instead
- baseUrl is static/js
some updates to login redirects

/anything served by Hub redirects to /hub/anything

/hub/user/[user] spawns single-user server and redirects to it
add basic home, admin pages

- index no longer requires login, it's just a big fat login button
- home has stop, start, admin buttons
- admin doesn't do anything yet
- add jhapi javascript client for REST API
Merge pull request #30 from minrk/restapi

getting started with REST API