Add pipeline support

[markstory]

Looking good!

[markstory]

Thanks! Tagged and packaged into 0.5.0

[messense]

I think maybe we could make the pipeline API more like redis-rs:

extern crate statsd;

use statsd::Client;

let client = Client::new("127.0.0.1:8125", "myapp").unwrap();

let mut pipe = statsd::pipeline();
pipe.incr("some.counter");
pipe.decr("some.counter");
pipe.send(&client);

[markstory]

@messense That sounds like a simpler API :+1:. Did you want to do that change or should I do it?

[messense]

@markstory I don't have much time now. You should do it. :-)

[markstory]

@messense Ok.