feat(client): impl Sync for Client

Connector::connect already used &self, and so would require
synchronization to be handled per connector anyway. Adding Sync to the
Client allows users to setup config for a Client once, such as using a
single connection Pool, and then making requests across multiple
threads.

Closes #254

BREAKING CHANGE: Connectors and Protocols passed to the `Client` must
  now also have a `Sync` bounds, but this shouldn't break default usage.

examples/client.rs

@@ -20,7 +20,7 @@ fn main() {
         }
     };
 
-    let mut client = Client::new();
+    let client = Client::new();
 
     let mut res = client.get(&*url)
         .header(Connection::close())

examples/client_http2.rs

@@ -21,7 +21,7 @@ fn main() {
         }
     };
 
-    let mut client = Client::with_protocol(h2::new_protocol());
+    let client = Client::with_protocol(h2::new_protocol());
 
     // `Connection: Close` is not a valid header for HTTP/2, but the client handles it gracefully.
     let mut res = client.get(&*url)