Make time configurable

Adds the notion of current time to the config object. It will default to allowing PHP to come up with a current DateTime object, but you can do whatever you like in your setup (such as querying your database).

Example:

```
ActiveRecord\Config::initialize(function($cfg){
       $row = $database->execute("select now() as current_ts");
	$cfg->set_current_time(new \ActiveRecord\DateTime($row['current_ts']));
});
```