Implement temporaryUrl method

[fosron]

It was mentioned previously (#35), but now the official adapters like S3 and Rackspace have the capabilities built-in as:

\Illuminate\Filesystem\FilesystemAdapter::temporaryUrl

public function temporaryUrl($path, $expiration, array $options = [])
    {
        $adapter = $this->driver->getAdapter();

        if ($adapter instanceof CachedAdapter) {
            $adapter = $adapter->getAdapter();
        }

        if (method_exists($adapter, 'getTemporaryUrl')) {
            return $adapter->getTemporaryUrl($path, $expiration, $options);
        } elseif ($adapter instanceof AwsS3Adapter) {
            return $this->getAwsTemporaryUrl($adapter, $path, $expiration, $options);
        } elseif ($adapter instanceof RackspaceAdapter) {
            return $this->getRackspaceTemporaryUrl($adapter, $path, $expiration, $options);
        } else {
            throw new RuntimeException('This driver does not support creating temporary URLs.');
        }
    }

It should be possible to implement this in the adapter for GCS.