Update S3 external library

Summary:
This fixes at least two issues with the S3 library on newer versions of cURL/PHP:

  - NOTICE: PHP message: [2013-07-02 22:15:54] ERROR 8: curl_setopt(): CURLOPT_SSL_VERIFYHOST with value 1 is deprecated and will be removed as of libcurl 7.28.1. It is recommended to use value 2 instead at [/core/lib/phabricator/externals/s3/S3.php:1744]
  - `$this->request->body` was appended to without initializing it, which rasies an error on PHP 5.5.0.

I looked over the rest of the changes briefly and they all seem reasonable-ish.

Test Plan:
  - Uploaded a file to S3.
  - Downloaded a file from S3.
  - Deleted a file from S3.
  - Checked error logs for anything suspicious.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6349