Misc renames (#202)

This patch renames a number of types and functions making
the API more consistent.

* `Server` -> `Connection`
* `Client` -> `SendRequest`
* `Respond` -> `SendResponse`.

It also moves the handshake fns off of `Connection` and make
them free fns in the module. And `Connection::builder` is removed
in favor of `Builder::new`.

tests/support/prelude.rs

@@ -3,9 +3,9 @@
 pub use super::h2;
 
 pub use self::h2::*;
-pub use self::h2::client::{self, Client};
+pub use self::h2::client;
 pub use self::h2::frame::StreamId;
-pub use self::h2::server::{self, Server};
+pub use self::h2::server;
 
 // Re-export mock
 pub use super::mock::{self, HandleFutureExt};