Date-based filtering

This adds a `DateConditions` module, and implements it for `Projects#index` for updated_at filtering.

The module adds a scope to a model, which takes params and a type. The params it looks for are `*_min` and `*_max` of the supplied type, so `updated_at_min` for example. It then returns a filtered set of results based on the supplied params.

Should make syncing easier ;)  We use this in Unicorn for clients fetching posts and it works well.

@jules2689 @joshpc