users can delete their own accounts

closes #195 

I put the delete button on the user's show page. I'm not really sure where the ideal place to put it is though. I was wondering if maybe the delete button should take them to another page and get them to agree again there or something... right now there's just the standard 'are you sure you want to delete your account' popup. 

I tried looking on facebook for an example but... I can't for the LIFE of me figure out how to deactivate my fb account so that was not helpful. 
Small warning: this will fail when current_person is nil, which happens when someone looks at that page without being signed in. The controller currently prevents this by only allowing signed in users, so it can not happen right now. But maybe someone changes the controller in the future (I know, one should not program for some vague anticipation of the future, but still, in this case I think it would be justified to make the code here less brittle). Also, it's good to be consistent with the other views - so I'd suggest to add something like `if current_person` in front of this check.