Added initialize method for initiliazing S3 from a config file

[gabrieljenik]

Added initialize method for initiliazing S3 from a config file.

Usage:
$this->load->library('s3');
$this->s3 = new s3();
$this->s3->initialize($config); // Config can be a custom array. If null, config will be taken from config file.

Sample config file: (to be placed under config/s3.php)

    <?php
    /*
    | -------------------------------------------------------------------
    | Amazon S3 Configuration
    | -------------------------------------------------------------------
    */

    $config["accessKey"] = "AKIAJHFQ7V53XXI5CYTA";
    $config["secretKey"] = "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";
    $config["useSSL"] = FALSE;
    $config["endpoint"] = 's3.amazonaws.com';