Change pre_render and post_render callbacks to accept callables

Now we have moved to psr-0 and make heavy use of classes/namespaces etc... it would make sense to allow the '#pre_render' and '#post_render' callback arrays to process any php callable and not just a function. This would promote better code organisation. For a good example of this, see views_handler_field_custom_pre_render_move_text in views.module.