Building apps with Rwf guide, request session not optional, better ORM interface, other fixes

[levkk]

Breaking changes

Request::session() now returns the Session instead of Option<Session>. This reinforces the guarantee that all Rwf requests will have a user session, even if the browser doesn't respect cookie settings.

Features

1. Added hash and hash_validate using Argon2. argon2 crate added as dependency to rwf.
2. Starting guide on building applications with Rwf.

Other changes

1. Print a warning if the config file contains errors.
2. Generalized the connection object passed into ORM methods when executing queries. Allowed options now include the connection pool itself. This allows callers to avoid checking out a connection manually, and prevents common mistakes like checking out multiple connections.
3. Implemented Option<T: Model> for ToTemplateValue, allowing to pass in optional results from ORM directly into the template.
4. Allow trailing comma in render! macro when specifying response code.