Feature/multi upload dirs

I added a way for users to upload directories to s3 rather than just the parent directories of individual files. For example. If I have a css file(s), *.css, and they get picked up by gulp.src, then just the immediate folder the reside in will be transported to s3, if you have more than one directories to send to s3. Now, if you specify directories as a third (optional) parameter when using this plugin, the directory structure will be preserved when transporting more than 1 directories to a bucket.