Update README and Playground to reflect actual expression building

Signed-off-by: Stephen Celis <stephen@stephencelis.com>
Add Row object

Behaves like a dictionary, mostly, but provides typed access to a row's
values. Query objects yield them on iteration/access:

    let email = Expression<String>("email")
    let users = db["users"]

    for user in users.select(email) {
        if let email = user[email] {
            // email is String
        }
    }

Signed-off-by: Stephen Celis <stephen@stephencelis.com>