Add basic support for Last-Modified HTTP header.

```Rack::Scaffold``` checks if the used adapter does timestamping, and if so, uses
the most recent value of a generic ```updated_at_field``` for the
Last-Modified header.
Allow enabling and configuring of Sequel Timestamps plugin

CoreData adapter now looks for a :timestamps option and uses it to
configure the Sequel::Plugins::Timestamps plugin.

This allows a Helios user to do something like this:

service :data, model: Dir['*.xcdatamodel*'].first, timestamps: {create: :createdAt, update: :updatedAt}