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`.

README.md

@@ -51,7 +51,7 @@ Next, add this to your crate:
 ```rust
 extern crate h2;
 
-use h2::server::Server;
+use h2::server::Connection;
 
 fn main() {
     // ...