Make sure the functions are called statically

Hey @paulgibbs,

We're using Welcome Pack on a client project at the moment using php 5.4 and the following warnings are thrown:

```
Strict Standards: call_user_func_array() expects parameter 1 to be a valid callback, non-static method DP_Welcome_Pack::setup_admin() should not be called statically in /vagrant/wp/wp-includes/plugin.php on line 470

Strict Standards: call_user_func_array() expects parameter 1 to be a valid callback, non-static method DP_Welcome_Pack::check_installed() should not be called statically in /vagrant/wp/wp-includes/plugin.php on line 470
```

I've attached the easiest PR I've ever had to do ever :smile:

Thanks for making Welcome Pack :+1: