Add 2 new webservices:
  - ws=permalink : return link description (json) given a permalink, if found; 404 else
  - ws=link      : return link description (json) given an url, if found; 404 else

i.e:
  http://sharli.uri/?ws=permalink&term=kaJ9Rw
  http://sharli.uri/?ws=url&term=http:%2F%2Fsebsauvage.net%2Fwiki%2Fdoku.php%3Fid%3Dphp:shaarli

  both returns:
    {"title":"Shaarli - sebsauvage.net","url":"http:\/\/sebsauvage.net\/wiki\/doku.php?id=php:shaarli",
     "description":"Welcome to Shaarli ! This is a bookmark. To edit or delete me, you must first login.",
     "private":false,"linkdate":"20110914_190000","tags":["opensource","software"],"timestamp":943916400}