add current page attribute to method views_json_get

Hello, I used the method views_json_get, and I think that a quick and good enhancement for this method would be to support pagination.

Here is how to do it :

1) change the method signature from

views_json_get($name, $display_id = 'default', $args = array(), $raw = FALSE)

to

views_json_get($name, $display_id = 'default', $args = array(), $raw = FALSE, $current_page = 0)

2) just add after

 if (!is_object($view)) return NULL;

this single line

  $view->set_current_page($current_page);