New method RemotePathOutdated and bugfixes

* New method: RemotePathOutdated: Check if a given path is older
  in gdrive than a localFile. Useful to avoid unnecessary copies.
* Move: modified to remove the destination of a move if it's a
  directory. Also remove the old dstPath from the cache upon delete
  (if the next call fails, the cache will be correct.
* Insert: Now by default sets the just inserted file's ModifiedDate
  to the mtime of the local file.
* SetModifiedDate: Bugfix: Force the ASCII representation of the
  date to always contain nano seconds. Otherwise, Gdrive will return
  a 400 when trying to set the file's date.
Bugfix on Move

We now preemptively remove the entry from the cache if move fails.
This guarantees that even if the move fails and we return immediately,
the cache will be in a safer state.