S3 temp signed URL support.

Introduces AWSS3BlobRequestSigner, which reuses the
RequestAuthorizeSignature filter for most of the heavy lifting.
Other implementation details based on [1].

Tested with AWSS3BlobSignerLiveTest, in particular,
testSign(Get|Put)UrlWithTime.

Closes JCLOUDS-200

[1]
http://s3.amazonaws.com/doc/s3-developer-guide/RESTAuthentication.html