[Sync] Setup smart sync with dropbox

AiCloud now could do smart sync with dropbox, and here are steps:

1. Enable cloud disk and smart sync.

2. Select dropbox to create a new sync profile.

3. Enter related profile information and Authenticate with Dropbox.

Folder is to select a specific path on your cloud disk to do sync.
Rule:

• Sync: Files on both side will stay up to date. 
• Download to USB disk: Files(include new files added) on web storage will be download to USB hard drive. Files on USB hard drive will NOT be upload to web storage. 
• Upload to Cloud: Instead of download to USB disk, files will be upload to web storage and will NOT download anything to USB hard drive.

To authenticate with dropbox, please click "Authenticate" button and there will be a pop-up window, and you will be asked to enter dropbox account and authenticate directly with dropbox server.