Sync improvements

This is my first open source code submission so please let me know if I am doing anything wrong.

I added two features to sync_folder_to_container.
1) Instead of downloading the data for each object it downloads a list and uses that to compare the md5 against. This speeds things up a lot.
2) I added the ability to specify a prefix to be used during the sync. This allows you to sync a folder into just a part of the container instead of the entire container. This also lets you do an incremental sync if your files to be synced are in different folders.

I added verbose logging to sync_folder_to_container while I was in there. #250