Add a recursive file copy function

[erickt]

Cargo needs a recursive copy function in order to install the OS X .dSYM files.

[kud1ing]

Is this still true for rustpkg? Ping @z0w0

[z0w0]

I'm not sure. I need @erickt to clarify what he means.

[bstrie]

I'm seeing the existence of os::copy_file(), I presume what this issue needs is os::copy_dir() ? @catamorphism, do you know if this requirement is still necessary for rustpkg?

[catamorphism]

@bstrie I added os::mkdir_recursive() recently, which I think is what this issue is asking for. So I'm closing this.