Add confirmation of database delete, fix delete bug

This is to try and prevent accidental / confused deletion, as experienced previously by IDOC.
See issue #23 for details on UX issue and fix.

Also, `DeleteFile` returns `True` if successful, but in the script interpreted as failure.  Fixed that.
http://www.jrsoftware.org/ishelp/index.php?topic=isxfunc_deletefile

@ruimalheiro can you take a look?