Add secret to constructor
Create token from secret if secret is available, else from serialized $_SERVER
Merge branch 'feature/csrfguard-enhanced-token' into develop
Allow enhanced security by providing secret key to CsrfGuard to base token on instead of $_SERVER which is slightly predictable.
Unset token after it has been used
Move token creation after validation check

When the token has been used and thus unset, it needs to be created
again in case we want to use it in a form.
Check if key exists in session before retrieving it
Merge branch 'feature/csrfguard-refresh-token' into develop
Make sure a new token is generated after a token has been consumed.
Tokens should not become guessable because of repeated usage.