In place record update

[groue]

This pull request introduces a new way to perform database updates: updateChanges(_:with:).

// Mutates the record according to the provided closure, and then, if the
// record has any difference from its previous version, executes an
// UPDATE statement so that those differences and only those difference are
// saved in the database.
if var player = try Player.fetchOne(db, key: 42) {
    try player.updateChanges(db) {
        $0.score += 10
        $0.hasAward = true
    }
}

updateChanges(_:with:) returns whether the record was changed or not:

let modified = try player.updateChanges(db) {
    $0.firstName = "Arthur"
    $0.lastName = "Smith"
}
if modified {
    print("Player was modified, and updated in the database")
} else {
    print("Player was not modified")
}