Ignore the $WhatIfPreference and $ConfirmPreference variables

I use psake for deployments so its quite nice to be able to use -whatif to see what would happen if you pulled the trigger. Currently psake gets in the way a little too much and the script variables end up not being set correctly so the -whatif misreports.

I've added -WhatIf:$false -Confirm:$false to the relevant Set-Item calls in psake so the build script can execute correctly.

Also fixed: https://github.com/psake/psake/issues/57 (the error message is only swallowed if psake is not executing a nested build.)