Merge webhdfs to 0.23 (HDFS-2316)

[Tsz Wo Sze]

Hi,

WebHdfs is a complete FileSystem implementation  for accessing HDFS over HTTP.  
It has a FileSystem implementation (WebHdfsFileSystem, webhdfs://) and a REST 
API.  See HDFS-2316 for more details.

WebHdfs was committed to trunk, 0.20-security branch and 0.20.205 branch.  I 
propose merging it to 0.23.

Regards,
Tsz-Wo