Some little modifications in S3 Task

Hi there,

I made two little modifications: 

* Added overwrite attribute to S3Task. If you set true, it will avoid the checking and execute the upload overwriting the file which is in the bucket

* Added key attribute. If you use the key, you'll be to create "virtual" folders inside your bucket. 

As follows:

<amazon-s3
			accesskey="${access.key}"
			secretkey="${secret.key}"
			bucket="s3-gaia-application-drive"
			file="${gaia.dir}/gaia-release.zip"
			key="old-releases/gaia-release-${build.date}.zip"
			overwrite="true"
		/>

Thanks,

Armando