confwiz: add command line options

confwiz: add command line options

adds command line option parsing to the confwiz.

the previous --uninstall option behaves as before, but it parsed
more explicitly with the twisted usage library.

added is a --server option, which controls which web site the
backend script for configuration is to be found on. (it is looked
for at /native_client.php on the given server) this option can be
used on conjunction with --uninstall to control where the uninstall
is recorded

Options:
  -u, --uninstall  record uninstall
  -s, --server=    url of server to contact
                   [default: https://beta.allmydata.com/]

e.g. confwiz.py -s https://www-test.allmydata.com/