Update class

Hi, I just found your class and I think it's awesome... I made some little changes you could like, so here's the changelog :

+ Renamed $website to $url in `request`
+ Added `CURLOPT_SSL_VERIFYPEER = false` option to curl request
+ Closed curl session before returning (code after return is never
executed)
+ Added array for easy maintenance of signin errors
+ changed `is_premium` url
+ `is_premium` return now boolean
+ `get_skin` return now the skin url
+ removed duplicate code in `render_skin` and user early return to
simplify code
+ removed `?>` end tag, [see
doc](http://php.net/manual/en/language.basic-syntax.phptags.php)