Please support creating temporary directories with mode 0700

[joshtriplett]

tempfile creates temporary files with mode 0600, but doesn't seem to restrict permissions on temporary directories at all. I'd like to create a temporary directory with mode 0700 (and not rely on the current umask to restrict write access). Ideally, I'd like tempfile to do so by default, for security reasons; if there's some reason the default permissions can't change, then I'd like to have an option to set them.

[Stebalien]

See #61. Basically: needs implementation.

[lucab]

I also have a usecase where explicitly setting the permission mask (0664 in my case) without relying on the default umask would be helpful.
I think that having a new setter method on the builder, taking a proper Pemissions argument, would be great.
#61 is not currently going in that direction due to shortcomings on non-Linux platforms.

[Stebalien]

Implemented.