Allow for changing user passwords

[dudefellah]

This PR should allow for both the creation and update of Samba user passwords. In addition, it stores sensitive passwords in temporary files instead of printing them out on the command line where they could be seen in a process listing.

[mkfrey]

I like this pull request. This also solves the no_log problem in the pdbedit command exection (and therefore #53) and also does not cause accidential variable expansion when the password contains $ characters.