addr config fields should accept hostnames

[klingerf]

Right now the addr fields in config files parse their values as inet socket addresses. We'll eventually need those to accept hostnames as well. Right now, addr: namerd:4180 throws the following error:

thread 'main' panicked at 'configuration error: Error { repr: Custom(Custom { kind: InvalidData, error: Message("invalid IP address syntax", Some(Pos { marker: Marker { index: 82, line: 6, col: 8 }, path: "proxies[0].namerd" })) }) }', /buildslave/rust-buildbot/slave/stable-dist-rustc-linux/build/src/libcore/result.rs:868