Add createPrefixedReadStream

Hello @x-ma, 

Please review the following commits I made in branch 'es-prefixed-read'.

4a20a64971249a81b22fe2004d3a02590075c684 (2014-02-18 17:37:13 -0800)
Add createPrefixedReadStream
Downloads all S3 objects that match a prefix as a single stream. Useful for tools that create multiple S3 files for a single data source like Redshift's UNLOAD command.

R=@x-ma