Add exec command button in tutorials

If you add a html `<button data-exec="myCustomCommand">customCommand</button>` then clicking that button execute the sbt command in `data-exec`.

![screen shot 2014-10-07 at 12 32 20](https://cloud.githubusercontent.com/assets/197573/4540969/bd144ea8-4e0d-11e4-8b2b-bd003eff61df.png)


I guess we need to talk about that feature again, I believe it slipped out.