Use HTTPClient instead of file_get_contents

Allows us to catch exceptions like 404's and thus prevents stuff like:

```
{"reqId":"fnmS0hc+fL4+WXKAruo5","remoteAddr":"192.168.39.100","app":"PHP","message":"file_get_contents(http:\/\/ckamm.blaucloud.de\/\/status.php): failed to open stream: HTTP request failed! HTTP\/1.1 404 Not Found\r\n at \/var\/www\/owncloud\/apps\/files_sharing\/lib\/external\/storage.php#260","level":3,"time":"2016-03-08T21:00:33+00:00","method":"PROPFIND","url":"\/remote.php\/webdav\/"}
```