Added ScriptPack<T> base class

Tested it with the `ScriptCs.WebApi` and it works great :smile:
```csharp
public class ScriptPack : ScriptPack<WebApi>
{
    public override void Initialize(IScriptPackSession session)
    {
        var namespaces = new[]
        {
            "System.Web.Http", 
            "System.Web.Http.SelfHost", 
            "System.Web.Http.Dispatcher"
        }.ToList();

        namespaces.ForEach(session.ImportNamespace);
    }
}
```

Fixes #285.