This commit includes the following additions:
* A `session` example was added.
* `Config::take_session_key` was removed.
* If a `session_key` is not supplied, one is automatically generated.
* The `Session` type implements signed, encrypted sessions.
* A `Session` can be retrieved via its request guard.
This commit involves several breaking changes:
* `session_key` config param must be a 256-bit base64 encoded string.
* `FromRequest` is implemented for `Cookies`, not `Cookie`.
* Only a single `Cookies` instance can be retrieved at a time.
* `Config::take_session_key` returns a `Vec<u8>`.
* `Into<Header>` is implemented for `&Cookie`, not `Cookie`.
A few interesting notes on this breakage:
* `Cookie` how has a lifetime. It should be `'static'` everywhere.
* The `SetCookie` header is no longer reexported.
* Instead, `Cookie` implements `Into<Header>` for Set-Cookie.
Sergio Benitez