Added rich wrapper for JsField with specialised field getters

When writing a custom `JsonFormat` the currently proposed way of using pattern matching/extractors on `JsObject.getFields` is elegant and concise especially when all the fields are required.

However we often meet the situation where, even though the fields in the domain object are required, we can provide default values for some of these fields, thus making some part of the incoming json optional. For example, a `User` class might require a language and timezone parameters but we we can use the system defaults if the incoming json does not specify these fields.

I experimented with multiple options that would make easier the writing of `JsonFormat`s that allow optional fields and I think the best one is to have a rich wrapper for `JsObject` that offers specialised getters. If these getters return `Either[String,T]` a for comprehension can be used on their `Right` projection. 

This also has the added advantage of not polluting the base implementation of `JsObject` and can be used only when facing the optional fields case.

The whole thing is demonstrated in the `RichJsObjectSpec `.