feat(client): update construction of Clients

- `Client::new()` no longer needs a `Handle`, and instead makes use of
  tokio's implicit default.
- Changed `Client::configure()` to `Client::builder()`.
- `Builder` is a by-ref builder, since all configuration is now
  cloneable pieces.

BREAKING CHANGE: `Client:new(&handle)` and `Client::configure()` are now
  `Client::new()` and `Client::builder()`.

examples/client.rs

@@ -31,7 +31,7 @@ fn main() {
     }
 
     tokio::run(lazy(move || {
-        let client = Client::default();
+        let client = Client::new();
 
         let mut req = Request::new(Body::empty());
         *req.uri_mut() = url;