Added upload directory option to s3

Hello!

I was just adding another feature to s3 uploading. Currently, s3 uploading uploads to the root directory to s3 and overwrites any files with the same name. This allows people to upload to a specific directory. I'm intending this to mostly be used with the TRAVIS_BUILD_NUMBER environmental variable or something like that.

This allows people to a upload to a specific folder in s3. Defaults to root directory.