Unable to set handle class to DBI::db

If pointed to use `handle_class: 'DBI::db'`, the plugin tries to require `DBI/db.pm` module and Dancer raises an exeption because DBI::db package located inside DBI.pm module.

At the moment I dont know how to handle this. May be a hashref of classes and its packages to load will be used. E.g.

```
$special = {
'DBI::db' => 'DBI.pm',
'DBD::Proxy::db' => 'DBD/Proxy.pm',
...
};

if (exists $special->{$handle_class}) {
$package = $special->{$handle_class};
}
else {
$package = $handle_class;
$package =~ s{::}{/}g;
$package .= '.pm';
}

require $package;
```