Added a hook to post-process models

Here someting which was on my mind for a while, but I finally took the time to implement it. 

In #17 we talked a bit about some attributes with valuable metadata, like <code>System.ComponentModel.DataAnnotations.RequiredAttribute</code> or <code>Newtonsoft.Json.Serialization.JsonProperty</code>.

I suggest adding/using the following interface for this:
```csharp
public interface IModelPostprocessor
{
    void Process(SwaggerModel model, SwaggerModelData data);
}
```

These processors are invoked after a <code>SwaggerModel</code> is created, giving you the chance to modify it.

Example implementation:
```csharp
public class JsonNetModelPostprocessor : IModelPostprocessor
{
    public void Process(SwaggerModel model, SwaggerModelData data)
    {
        // Reflect the properties in data.ModelType to find 
        // JsonPropertyAttribute, and use this to modify
        // the model property's names for example
    }
}
```