Add file listing API

Implement listing the blob of a bucket. The request can be defined by the parameters "limit", "prefix", "sort".

* "prefix" list only blobs with given prefix
* "limit" return max limit blobs
* "+/-sort" sort the returned blobs in asc/desc order sort = {"lastModified", "key"}
Merge pull request #11 from soundcloud/list-api

Add file listing API