Provide methods to get the progress of a running upload to a blobstore

The jclouds library should provide a way to get progress information about a running upload to any blobstore (information like how many bytes where already transferred and optionally average speed of the transfer).

In JavaUrlHttpCommandExecutorService, the outputStream is already being wrapped in a CountingOutputStream, but there is no way to call the getCount() method from outside the method.

There should be some way to access the current count of bytes already sent to calculate the status of the upload. Additionally a way to get the average speed would be nice, but I guess this is more optional because you can calculate the speed on your own if you know how many bytes where already transferred.

Another option to solve this, is to provide access to provider-specific features like the ProgressListener in the S3-API (see http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/services/s3/model/ProgressListener.html), but I guess that not all vendors have something similar in their APIs, anyway the first approach would be a more generic one.