Fixed bug that cause some magnet links to fail opening

Found the bug that was causing some magnet link to not open.
URL escaping in TSTorrentFunctions.m `downloadEpisode: ofShow:` (line 102) should be done as:
``` objc
[NSURL URLWithString:[url stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding]
```
instead of
``` objc 
[url stringByReplacingOccurencesOfString:@" " withString:@"%20"]
```

Replacing whitespaces with "%20" could result in a malformed string not accepted by `URLWithString:` (in which case the return value is nil and clearly `openURL:nil` ends up in doing nothing).