added s3 URL as an optional parameter to S3 client

Summary:
The finagle-aws client has the s3 URL hardcoded as s3.amazonaws.com which
causes requests to return with HTTP 307 requiring a redirict to zone local urls e.g.
s3-us-west-2.amazonaws.com. A new client parameter called 'url' now allows setting
this url.
This still can cause 307s but much less frequently. I will add the ability to handle
307 to the archiver and will later merge the re-direct back into finagle-aws.

Test Plan: tested this with ino-analytics

Reviewers: tberman, peter, jyeo

CC: jimmy, justin

Differential Revision: http://phabricator.thefactory.com/D537