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. 
I changed it to `current_person.destroy`. However, I'm not too sure how to go about signing out the user. Devise allows me to do `sign_out current_person`, and if I put that after the destroy, I'm redirected to the root page and everything looks fine, but my registrations controller specs fail with the msg `undefined method `user' for nil:NilClass`, which is odd... since there's not a user anywhere in this project, so it must be something up the chain in Devise. Not sure where to go from here....