preserve background color after filtering

by saving the background color on the note model itself, the background
color is preserved after the filter is applied.
improve dev, build, and deploy process

- dev now uses livereload that observes the built app.js and index.html
  for changes
- remove app.js from repo since it is to be built whenever `npm install`
  is ran, in otherwords when we deploy to heroku
- use express for static serve, fewer LOC
move calling firebase.onAuth to ApplicationRoute#activate

Register the firebase onAuth callback on the Application Route's
activate hook so that we can use `this.transitionTo` instead of the
global `App.Router.router`.

The original solution was pretty hacky.
listen to css for livereload
add checkerboard and striped background patterns to notes

Add a BackgroundGenerator class and assign an instance of it to each
note. A new instance will randomly choose a color using the
`randomcolor` library and a random pattern, either striped or
checkerboard.

The instance exposes the `toCSS()` method that returns a string of the
css rules for the background pattern.
add polkadots background pattern
add example application for gh-pages. no auth required