Allow to define new Resolver and new Expectation easily.

[shulard]

Today Raven defines Resolver objects that will be used to enrich the Request body and parameters.

There are built in definition that can be used easily. However if you define some inside a specific project, it’s not really easy to add them. Of course it’s possible but not easy.

We might consider allowing external object that implement the corresponding interface to be registered easily.

That can also allow the ability to create raven plugins more easily.