Custom generators possibility added. Custom generators should be developed as following:
    - custom generators are plugins for Rhetos
    - custom generators are loaded and instanced through Autofac, therefore, generators have to implement Rhetos.Generator.Interfaces.IGenerator interface and Export them selves as MvcGenerator.cs does.

Generator for MvcModel (including DataAnnotation attributes) is added, and included in CreateInstallationPackage.bat.