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. 
Is there a reason for finding the person again and putting it into `@person`, instead of just calling `current_person.destroy`? This is not a rethorical question, there might be a reason, I'm really wondering about it. But if not, then it's unnecessarily redundant.

Also, maybe it would be good to explicitly sign out the user - there must be some kind of method provided by devise that signs out the user, that could be called here. Otherwise, the reference to the now destroyed user (probably the id) will still remain in the session, which somehow feels a bit dirty, even though it might currently behave as expected (show a signed out user).