* Define Mover interface to move a single object
* Define DirMover interface to move a directory
* Implement DirMove operation
* Add `rclone move` command
* Tests for Dir Move
To Do
* Implement Move, DirMover in local, drive, dropbox
* unit test for Mover
* unit test for DirMover
This means that dropbox no longer stores MD5SUMs and modified times.
Fixup the tests so that blank MD5SUMs are ignored, and that if
Precision is set to a fs.ModTimeNotSupported, ModTimes can be ignored too.
This opens the door for other FSs which don't support metadata easily.
