This is a (minor) breaking change. If `rocket.launch()` is the last expression
in a function, the return type will change from `()` to `LaunchError`. A simple
workaround that preserves the previous functionality is to simply add a
semicolon after `launch()`: `rocket.launch();`.
resolves#34
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.
Sergio Benitez