pingback.ping:
    Pingback server function.

Requires URL of pinger resource and link of pinging resource.

Determinition of pingable object by it's url is done by setting
settings.PINGBACK_SERVER in the following format::

    {
    'post_detail': 'pingback.getters.post_get',
    }

Where:
 - 'post_detail' is name of url
 - 'pingback.getters.post_get' is name of function, which will return
   object using parameters, resolved by URL.