Make upload destination an optional argument

Fixes #11

In "kamaki file upload", the destination path is now an optional
argument, set to be equal to the base name of the source path, by
default.
Separate empty dir, empty container, etc. errors

This fix affects "kamaki file list [/CONTAINER[/PATH]]" command

Before this fix, if there were no results to show to the user, an
"empty container" notification would appear.
Now, the command distinguishes between the various cases:
 - empty container
 - empty directory
 - non-existing object
 - non-directory object
and notifies appropriately.