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()`.

src/client/pool.rs

@@ -503,11 +503,12 @@ mod tests {
 
     #[test]
     fn test_pool_timer_removes_expired() {
+        use std::sync::Arc;
         let runtime = ::tokio::runtime::Runtime::new().unwrap();
         let pool = Pool::new(true, Some(Duration::from_millis(100)));
 
         let executor = runtime.executor();
-        pool.spawn_expired_interval(&Exec::new(executor));
+        pool.spawn_expired_interval(&Exec::Executor(Arc::new(executor)));
         let key = Arc::new("foo".to_string());
 
         pool.pooled(key.clone(), 41);