Signing out bad user

This checks if the API response if a 401 (Unauthorized) if it is, then it signs the user out.

This _isn't_ something which is going to be that regular, but still, I'm unsure what the correct iOS way to do this operation is as the error message could be a bit more errory.

Anyway, the functionality itself works.

NOTE! The Snowpool API itself has changed to send 401's rather than (the incorrect) 400s it was sending.  I have updated this on the staging api http://snowpool-development.herokuapp.com 

To login on staging:
user : testing@snowpool.org
password: password