Now supports "persistence mode", to persist a single CSRF name/value pair throughout the life of a user's session. Added the following methods:

• protected getLastKeyPair - gets the most recently generated key/value pair from storage.
• protected loadLastKeyPair - gets the most recently generated key/value pair from storage, and assign it to $this->keyPair.
• public setPersistentTokenMode
• public getPersistentTokenMode

Note that if CSRF token validation fails, then the token should be renewed regardless of the persistence setting.

The methods getTokenName and getTokenValue now return null if $this->keyPair has not yet been set.

• testPersistenceModeTrueBetweenRequestsArray - Token should persist between requests after initial creation, when stored in an array.
• testPersistenceModeTrueBetweenRequestsArrayAccess - Token should persist between requests after initial creation, when stored in an ArrayObject.
• testPersistenceModeFalseBetweenRequestsArray - Token should be changed between requests, when stored in an array.
• testPersistenceModeFalseBetweenRequestsArrayAccess - Token should be changed between requests, when stored in an ArrayObject.
• testUpdateAfterInvalidTokenWithPersistenceModeTrue - New token should be generated after an invalid request, even if persistence mode is enabled.