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

.travis.yml

@@ -102,4 +102,4 @@ env:
     - RUST_BACKTRACE=1
 script:
   - cargo build $FEATURES
-  - cargo test -v $FEATURES -- --test-threads=1
+  - cargo test -v $FEATURES --features __internal_proxy_sys_no_cache -- --test-threads=1