Cater for different server variables on Windows

`$_SERVER['HOME']` doesn't exist on Windows, instead you have to join `$_SERVER['HOMEDRIVE']` and `$_SERVER['HOMEPATH']`. 

This PR changes the `Application` class to handle this, preventing a notice being issued when running any command on Windows.

(I'm not aware of any other OSes that don't provide `$_SERVER['HOME']`, though in theory there might be some...)