Make zmq compatible with upcoming Rust version (>0.6)

The upcoming Rust version is for some parts of the Rust language not compatible with older versions:
 - "extern mod <ident> { ... }" is invalid syntax.
 - Rename str/vec::from_slice to str/vec::to_owned
 - Remove deprecated `ptr::addr_of'.

I compiled the zmq bindings successfully with this Rust version:
$ rustc -v
rustc 0.6 (2d28d64 2013-05-17 15:52:25 -0700)
host: x86_64-unknown-linux-gnu