Phalcon\Session\Adapter and Phalcon\Session\Bag improvements

Phalcon Kernel:
  * fix `phalcon_hash_get()` when type is `BP_VAR_RW` or `BP_VAR_W`;
  * `null` offset passed to `phalcon_update_property_array()` appends a new element;
  * added `object_properties_init()` for PHP 5.3

Phalcon\Session\BagInterface:
  * added `remove()` method to the interface

Phalcon\Session\Adapter:
  * implements ArrayAccess, Countable, IteratorAggregate
  * optimized using object handlers
  * __get() and offsetGet() return values by reference to allow indirect modification (see #1895)

Phalcon\Session\Bag:
  * implements ArrayAccess, Countable, IteratorAggregate
  * __get() and offsetGet() return values by reference to allow indirect modification (see #1895)