split Client into (Client, Connection) (#107)

The Connection type is a `Future` that drives all of the IO of the
client connection.

The Client type is separate, and is used to send requests into the
connection.

tests/support/mod.rs

@@ -1,5 +1,11 @@
 //! Utilities to support tests.
 
+#[cfg(not(feature = "unstable"))]
+compile_error!(
+    "Tests depend on the 'unstable' feature on h2. \
+    Retry with `cargo test --features unstable`"
+);
+
 pub extern crate bytes;
 pub extern crate env_logger;
 pub extern crate futures;