Update JWE.php

Modified decrypt method to accept callback as $private_key_or_secret parameter's value.
Fixes issue #5

Usage Example:
```php
$jwt = JOSE_JWT::decode($jwe->decrypt(function($kid){ 
    if($kid == 'foo') {
        return file_get_contents(__DIR__ . "/../../foo.rsa");
    }

    return file_get_contents(__DIR__ . "/../../default.rsa");
        
})->plain_text);
```