First draft of an absolute-newbie tutorial

I had a lot of problems getting off the ground with the guides as published on emberjs.com, so I plodded my way through the extant starter kit and put together guides/getting-started/your-first-ember-app.md.

It assumes you know JavaScript, but *nothing* about Ember, and just wants to get feedback on something working in the browser.  I'm not quite happy with the ending, it sort of chops off in mid-flow, but I couldn't immediately decide where to take it next.

Hope it's useful, I've pushed it at another dev here for feedback and proof-reading.