Add documentation for listing API

Add parameter to define for the sorting criteria of the returned blobs. @xla 
All go code should be linted and vetted before submitted. Golint discourages the use of dot imports and you usually don't see it in any of our codebases. For more background [this](https://code.google.com/p/go-wiki/wiki/CodeReviewComments#Import_Dot) section is really helpful.