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. 
This sounds indeed odd. Feel free to ignore the `sign_out current_person` hint then, before it becomes an eternal yak shave. I tried to google shortly for the error message but didn't find anything obvious. If you want to explore it further, I suggest looking at the stack trace of the error, and then into the code from devise where it happens.