Csrf prevention

Added simple csrf prevention to fix issue #5 (closed) :

• The server generates a random, unique csrf token, stores it in the session and appends the token on the map and admin pages
• The token is added to every POST, PUT and DELETE request inside 'x-csrf-token' header
• Likewise, for every POST, PUT and DELETE request, the server checks that the token stored in the session matches the token from the header