Add statistics

Hello.

I couldn't get over 5,000 blobs using this gem.

Because List Blobs API is limited to 5,000 items.

http://msdn.microsoft.com/en-us/library/windowsazure/dd135734.aspx

To get over 5,000 blobs requires 'marker' option which is returned by List Blobs API. 

I only need size of blobs within a container for now,
so I add 'WAZ::Blobs::Container#statistics' and 'WAZ::Blobs::Service#statistics'.
which sums size of blobs within a container partially.

Please let this branch pull into your repository or fix this issue in other way.

Best Regards,