Add an EnsureDirectoryExists Alias to CreateDirectory and not fail if it exists

[Redth]

Currently using CreateDirectory ("./tmp") throws an error if it already exists.

I'm finding a lot of times I end up with this code in my scripts:

if (!DirectoryExists ("./tmp"))
    CreateDirectory ("./tmp");

It would be very helpful to have an alias that does this for us:

EnsureDirectoryExists ("./tmp");

[gitfool]

I'm 'away from compiler' so I can't check, but ideally that should work for multiple levels of missing directories.

[gep13]

Fixed via 674420b