Added access.key and secret.key parameters available on s3cmd >= 1.5

Hi Whit Armstrong,

First of all I want to thank you for building this R package. I must say it's been very handy :)

I've faced a situation where I needed to retrieve a large number of files from different AWS accounts using AWS.tools when I suddenly realized that on early versions of s3cmd it wasn't possible to set an AWS accessKey/secretKey parameters. Fortunately they've added those parameters and I hope it gets on public repositories very soon. 

So I took the liberty (in a positive way) to update the AWS.tools package with  those parameters. I've also updated the s3.cmds.Rd man files with the corresponding documentation about them.

If you want to test it here is how:

    #installing the lastest version of s3cmd
    cd ~/Downloads/s3cmd-1.5.0-rc1/
    sudo python setup.py install
    sudo apt-get install python-dateutil

Example:
![screenshot from 2014-11-10 00 20 11](https://cloud.githubusercontent.com/assets/1011868/4970652/1cc5e1ac-6887-11e4-92a4-a31b44d0d7de.png)

If you think this is a fair improvement to AWS.tools, please accept my pull request.

Best Regards

Paulo Miguel Almeida Rodenas