Option to clear data after read()

Default behavior is still to persist data, but now option exists in Session::read() to purge data after retrieval.

Use case: using Session class for temporary storage only (i.e.: some ID or something that needs to be carried over after a redirect but not used after).

```php
$this->Session->read("some name"); // Default behavior, "some name" value persists after call

$this->Session->read("some name", false); // "some name" will be cleared after call
```