From<http::Response> for Response (#360)

This adds an implementation to convert a `Response` type from the `http`
crate to the `async_impl::Response` type. This is the first step to
allow us to convert `http::Response` objects to `request::Response`
objects

This also adds an extension trait for the `http::response::Builder`
type. The `http::Response` object does not provide a way to access the
"final" url that the response is derived from, so we can't easily
provide that in the `From<http::Response>` implementation. For users who
are manually constructing `http::Response` objects for use in tests,
etc, they can import this extension trait, which adds a `.url()` builder
method that will allow them to pass a `Url`, which we then convert to
our newtype'd Url and add to the `http::Response`'s `extensions`. Then,
when converting from `http::Response` to `async_impl::Response` we can
pull that value out of the `extensions` and use it to construct the
`async_impl::Response`

Closes #333

src/async_impl/mod.rs

@@ -2,7 +2,7 @@ pub use self::body::{Body, Chunk};
 pub use self::decoder::{Decoder, ReadableChunks};
 pub use self::client::{Client, ClientBuilder};
 pub use self::request::{Request, RequestBuilder};
-pub use self::response::Response;
+pub use self::response::{Response, ResponseBuilderExt};
 
 pub mod body;
 pub mod client;