Predicted content types

When uploading a file, this will add the predicted content type for the file using the python `mimetypes` module. Also added a `--no-content-types` option, which disables this. This is important because, for example, IE will not render CSS unless it has the correct content type, and the current version of boto-rsync does not set it correctly.

I couldn't manage to get it to do this when transferring a file from one S3 bucket to another.