Move exercise goals/descriptions to top.

For the most part, the content itself is the same. Also includes some CSS tweaks I added along the way.

This makes lessons easier to get started with – a student can get started trying to do something straight away if they're feeling confident, and it provides some context for the rest of the content they're reading. Thanks! :)

![screenshot_2014-06-29_3 34 48 pm](https://cloud.githubusercontent.com/assets/569817/3424680/1659da92-ffde-11e3-81c8-8038e413690c.png)