Fix typo when calculating download target path

Fixes #30
Check if container exists in upload and download

Fixes #31
Modify upload and download semantics to make sence

Uploads and downloads have the form
kamaki file ACTION SOURCE [TARGET]

Problem:
If the SOURCE is a path and TARGET is missing, kamaki used to preserve
the path structure on the destination e.g.,
$ kamaki file upload /tmp/some/file.txt
  create remote dir tmp
  create remote dir tmp/some
  upload /tmp/some/file.txt --> tmp/some/file.txt

Solution:
Use the basename of the SOURCE in this case e.g.,
$ kamaki file upload /tmp/some/file.txt
  upload /tmp/some/file.txt --> file.txt