Add keys endpoint "/keys/:id"

[rasj]

Add a endpoint to search by a key id.

$client = new \Gitlab\Client(GITLAB_URL);

$client->authenticate(GITLAB_KEY, \Gitlab\Client::AUTH_URL_TOKEN);

$key = $client->api('keys')->show(1); 
/*
 {
     "id" : 1,
     "title" : "A key",
     "key" : "key",
     "created_at" : "2016-01-01T01:00:00.000Z",
     "user" : {
          ...
     } 
 }
*/