Add a few hints that the tutorial glosses over

Just added the Unix command for `wget`. I actually had to Google that when I came across this, so I figured I'd save some other Mac users the trouble. 

Also added the reminder to `bundle install` after adding gems. If you're reading the tutorial, you probably know that, but there might be a lot of Ember developers who aren't as familiar with Rails who might forget.

Lastly, fixed a spelling error.

Thanks for the tutorial!