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.

src/client/pool.rs

@@ -98,7 +98,7 @@ impl<S> PoolImpl<S> {
 
 impl<C: NetworkConnector<Stream=S>, S: NetworkStream + Send> NetworkConnector for Pool<C> {
     type Stream = PooledStream<S>;
-    fn connect(&mut self, host: &str, port: u16, scheme: &str) -> io::Result<PooledStream<S>> {
+    fn connect(&mut self, host: &str, port: u16, scheme: &str) -> ::Result<PooledStream<S>> {
         let key = key(host, port, scheme);
         let mut locked = self.inner.lock().unwrap();
         let mut should_remove = false;