feat(error): add Ssl variant to hyper::Error

The errors from openssl were previously boxed into a
Box<std::error::Error>, which lost some specifics and made it difficult
to match against. To solve this, an `Ssl` variant is added to the
`Error` enum of hyper, and is returned when openssl returns specific
errors.

Closes #483

BREAKING CHANGE: Adds a variant to `hyper::Error`, which may break any
exhaustive matches.

benches/client.rs

@@ -79,7 +79,7 @@ struct MockConnector;
 
 impl net::NetworkConnector for MockConnector {
     type Stream = MockStream;
-    fn connect(&mut self, _: &str, _: u16, _: &str) -> io::Result<MockStream> {
+    fn connect(&mut self, _: &str, _: u16, _: &str) -> hyper::Result<MockStream> {
         Ok(MockStream::new())
     }