Added support for username based configs

Added support for username based configs (issue #169). When using .htpasswd files, just create a config_local.<username>.php file with the user specific configuration. Be aware a username can only contain a-z, A-Z, 0-9 and _- characters.

When using the cops basic authentication you need to add a master account into the config_local.php like:
```
$config['cops_basic_authentication'] = array('username'=>'master', 'password' => 'master-password');
```
And add the same line in your config_local.<username>.php file with the username account and the specific configuration options for this user.

Is this ok? and where do you want to have the docs for this feature? I was thinking about a line in the default config to a new wiki page?