remove plugin fails on wp 3.5

Removing plugin from older WP (where it can't be activated) fails with message:

```
Fatal error: Call to undefined function wp_is_writable() in .../wp-content/plugins/backupwordpress/functions/core.php on line 380
```

Easy solution would be to add:

```PHP
if ( !function_exists('wp_is_writable') ) {
// copy wp_is_writable function from /wp-includes/functions.php
}
```