Implement fallback if xattr is not available on the file system

Under Linux extended attributes are supported but not activated by default on most filesystems (ext2, ext3, ext4, ...).
So remounting the partition is necessary for Drive to work:

sudo mount -oremount,user_xattr /dev/sda3

This is added in the installation documentation for Linux https://github.com/nuxeo/nuxeo-drive/#ubuntudebian-and-other-linux-variants-client

The fact that Drive won't start without this option can be a problem.

Under OS X, HFS+ filesystems support extended attributes by default, but if a user decides to put the Drive files on another filesystem type this might not be the case, preventing Drive from working.

See http://en.wikipedia.org/wiki/Extended_file_attributes