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.

src/net.rs

@@ -317,7 +317,7 @@ impl NetworkStream for HttpStream {
 pub struct HttpConnector(pub Option<ContextVerifier>);
 
 /// A method that can set verification methods on an SSL context
-pub type ContextVerifier = Box<Fn(&mut SslContext) -> () + Send>;
+pub type ContextVerifier = Box<Fn(&mut SslContext) -> () + Send + Sync>;
 
 impl NetworkConnector for HttpConnector {
     type Stream = HttpStream;