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. 
Another thing: what about wrapping the if clause around the whole row div? Otherwise an empty unneeded div is added for the cases where the button is not displayed.