audiofield unable to locate jplayer, add BACKDROP_ROOT.

I found that even when placing the files where instructed, audiofield was not able to locate my jplayer library. It looks like this is the culprit:

before:
```php
$players_dir = config_get('audiofield.settings', 'players_dir');
```
after:
```php
$players_dir = BACKDROP_ROOT . config_get('audiofield.settings', 'players_dir');
```
