Add getUploadRatio() function to retrieve share ratio on current torrent object

Usage example:

// Getting a specific torrent from the download queue
$torrent = $transmission->get(1);

// Getting share ratio of the selected torrent
$torrent->getUploadRatio();  // Will return 0.5033 for example; If the return value is 1, it will say that the torrent is seeded 1 time
Merge pull request #29 from firesalp/patch-3