Allow values to insert to be passed as array of setters

Hello!

It can be helpful to build up the setters for a query dynamically. If Swift supported splatting, then the existing `insert` could be used (e.g. `insert(*[name <- "Alice", email <- "alice@mac.com"])`), but it doesn't (https://devforums.apple.com/message/970958).

This diff lets the columns to be built up dynamically. For example:

    var values = [name <- "Alice"]

    if shouldSetEmail {
        values.append(email <- "alice@mac.com")
    }

    if let insertedID = users.insert(values) {
        ...
    }

If you think this is a good idea, `update` should also be changed to allow an array of setters.