Don't start syncdaemon on windows vista or later if running with escalated privileges

[Roberto Alsina]

If the user starts syncdaemon with escalated privileges, the Ubuntu One and partial folders will have broken permissions.

There is no reason to actually do that, and on Linux we prevent running as root. Therefore, we should make that check crossplatform.

[Natalia Bidart (nataliabidart)]

In ubuntuone/platform/windows/os_helper.py, we have this code that should be completed:

def is_root():
    """Return if the user is running as root."""
    # TODO: Do check if we are running as admin
    return False