Rewrite with ember-cli and more idiomatic Ember

This is a rewrite of Ember beat with more idiomatic Ember patterns. The pub-sub mechanism has been replaced as the run loop now seems fast enough to trigger audio for a drum machine.

I've made some minor UI improvements too:

![image](https://cloud.githubusercontent.com/assets/2526/2936275/1c410a5a-d853-11e3-8056-a173b9bbf178.png)

 * [x] Remove old code
 * [x] Add rewritten ember-cli app
 * [x] Update readme
 * [x] Add Channel Component
 * [x] Play sound on step click
 * [x] Review code for possible simplifications