Upload file from stdin

This adds the ability to put a file from stdin using the following command syntax:

s3cmd put - s3://BUCKET_NAME[/PREFIX]/FILENAME

Unfortunately, the only way that I could find to do this without major refactoring is to buffer the entire chunk from stdin, so it may be a good idea to override the configured chunk size to the minimum 5MB.  This is the first time I've written any python, so if you have any suggestions to improve this, they're more than welcome.