fail! macro undefined / outdated

[r-arias]

hey! thanks for this, I want to use it to route an HTTPS stream through Tor:
I was getting the following error when compiling:

src/socks4a.rs:45:18: 45:22 error: macro undefined: 'fail!'
src/socks4a.rs:45             x => fail!("Unexpected status byte: {}", x)
                                   ^~~~
src/socks4.rs:48:18: 48:22 error: macro undefined: 'fail!'
src/socks4.rs:48             x => fail!("Unexpected status byte: {}", x)
                                  ^~~~
error: aborting due to 2 previous errors
Could not compile `rustsocks`.

I went ahead trying to fix it and got a whole lot of more errors, related to being written for an older version of Rust. I am creating a pull request to fix this.