consolidated getter/setter for userdata to just "data", also added deprecations

http://codeigniter.uservoice.com/forums/40508-codeigniter-reactor/suggestions/2162587-consolidate-userdata-flashdata-getters-and-setters

example new usage for userdata

```php

<?

// setting 
$this->session->data('key', 'information');

// set with array 
$array = array('key' => 'information') 
$this->session->data($array);

//getting 
$this->session->data('key');

// get all 
$this->session->data();

```