#149 Fixed NPE when trying BucketWebsiteConfiguration is absent

According to [AWS SDK Javadocs](http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/services/s3/AmazonS3Client.html#getBucketWebsiteConfiguration(java.lang.String)):

> **Returns:**
> The bucket website configuration for the specified bucket, otherwise null if there is no website configuration set for the specified bucket.

This should fix the NPE in #149, for buckets that don't have a website configuration.