upgrade to native-tls 0.2 + invalid certs (#325)

- Bumps `native-tls` dependency to 0.2 and adapt code accordingly
- Import code used from `tokio-tls` into `connect_async` and adapt dependencies accordinlgy
- Add an option for using `danger_accept_invalid_certs` inside the `Config` struct

src/lib.rs

@@ -145,8 +145,8 @@ extern crate serde_derive;
 extern crate serde_json;
 extern crate serde_urlencoded;
 extern crate tokio;
+#[macro_use]
 extern crate tokio_io;
-extern crate tokio_tls;
 extern crate url;
 extern crate uuid;
 
@@ -173,6 +173,7 @@ mod error;
 
 mod async_impl;
 mod connect;
+mod connect_async;
 mod body;
 mod client;
 mod into_url;