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/tests.rs

@@ -20,9 +20,9 @@ fn retryable_request() {
     let sock1 = connector.mock("http://mock.local");
     let sock2 = connector.mock("http://mock.local");
 
-    let client = Client::configure()
-        .connector(connector)
-        .executor(executor.sender().clone());
+    let client = Client::builder()
+        .executor(executor.sender().clone())
+        .build::<_, ::Body>(connector);
 
     {
 
@@ -66,9 +66,9 @@ fn conn_reset_after_write() {
 
     let sock1 = connector.mock("http://mock.local");
 
-    let client = Client::configure()
-        .connector(connector)
-        .executor(executor.sender().clone());
+    let client = Client::builder()
+        .executor(executor.sender().clone())
+        .build::<_, ::Body>(connector);
 
     {
         let req = Request::builder()