Enable "system" proxies by default (#683)

If no proxies are configured for a client, the environment (system) will
be inspected automatically to set up proxies.

Configuring a `Proxy` on a client or calling `no_proxy` will disable the
use of the automatic system proxy.

Closes #403

src/lib.rs

@@ -263,6 +263,9 @@ if_hyper! {
     #[macro_use]
     extern crate doc_comment;
 
+    #[macro_use]
+    extern crate lazy_static;
+
     #[cfg(test)]
     doctest!("../README.md");