Change to BucketStats and Bucket.Stats().

This commit pluralizes the BucketStat type to be BucketStats. This makes it
more consistent with the other Stats() calls. This commit also changes the
return type to a struct instead of a pointer.

Finally, this commit adds documentation to the fields of BucketStats.
Merge pull request #137 from benbjohnson/bucket-stats

Change to BucketStats and Bucket.Stats().