added app_pool identity

This will allow you to change the app_pool identity to any of the allowed accounts:
SpecificUser, NetworkService, LocalService, LocalSystem, ApplicationPoolIdentity

To use a custom account you will set this to SpecificUser and then set "pool_username" and "pool_password", otherwise you can set it to one of the built in accounts NetworkService, LocalService, LocalSystem, ApplicationPoolIdentity
since the attribute has a default, this will never not be run. how about removing the default on the attribute, that way we don't have to worry about breaking backward compatibility because this just won't run?