Add Stack Exchange onebox

This onebox works for any question on the `*.stackexchange.com` network as well as these other domains:

`stackoverflow.com`
`superuser.com`
`serverfault.com`
`askubuntu.com`

If you'd like to add more domains later (I couldn't think of anymore), you can easily stick them in [`DOMAINS`](https://github.com/chrishunt/discourse/blob/1fb0b424ed62a38120419abec93050fce6fc540a/lib/oneboxer/stack_exchange_onebox.rb#L5) and they'll start working.

Here are a few examples:


![onebox-stackoverflow](https://f.cloud.github.com/assets/65323/632769/813c5550-d1eb-11e2-8eed-3de2f3a77187.jpg)

![onebox-superuser](https://f.cloud.github.com/assets/65323/632771/89fd13dc-d1eb-11e2-892d-0e01851ad60a.jpg)

![onebox-math](https://f.cloud.github.com/assets/65323/632772/91761bea-d1eb-11e2-8ecc-63ab1498784d.jpg)