Mandatory user filters support + utility functions + plus 204 status handling.

The same as https://github.com/lkundrak/www-gooddata/pull/5
plus Lubo's fix of 204 status handling (see 3552db34cb464de8f2bc3663f2a9fd5ce1b7ee7a)

Usage:

$gdc->login($login, $passwd) or die "Unable to authenticate\n";
my $resp = $gdc->assign_user_filter (
$project, $username, $label, @values);

Only "attribute IN (value1, value2, ...)" filters are supported.