Remove partial files

[maxsatula]

If an error occurs during transfer, remove partially uploaded or downloaded file, either on the Oracle side or local

[maxsatula]

• [x] add a switch to keep partially downloaded/uploaded files
• [x] add a switch to allow file overwrite (issue #5)
• [x] add a switch for resume operation

[maxsatula]

Proposed command line options:

--resume (implies --keep-partial)
--keep-partial

[maxsatula]

Alternatively

  -c,  --continue                resume getting a partially-transferred file.