Use std::net::SocketAddr instead of address (String) and port (u16) in config

[jhpratt]

Is there any particular reason why this isn't being used currently? SocketAddr has been stable since 1.0.0, and allows for parsing/stringifying both IPv4 and IPv6 addresses (including ports). As a bonus, this would slightly reduce the amount of work on Rocket's side, since you wouldn't need to parse it yourself.

Needless to say this would be a breaking change, so 0.5 at the minimum.

[jebrosen]

#594 also changes address configuration, but in a very different way. I think it's at least worth considering SocketAddr and documenting why we do/don't use it, especially for a hyper 0.12 migration which would change a lot of the low level connection code anyway.

I do like the convenience of ROCKET_PORT=8080, and it might be tricky to support both ideas.

[jhpratt]

With regard to ROCKET_PORT, there is a SocketAddr::set_port that takes a u16. It could certainly be documented that environment variables override the inline configuration.

The changeover to hyper 0.12 is a good point, a large portion will likely have to be modified regardless.

[SergioBenitez]

Let's handle this in #17.