Add DiscriminatorValue override to AutoMapping

[hazzik]

Allows to override discriminator value of the subclass.

var autoMapper = AutoMap.AssemblyOf<ExampleClass>()
    .Where(t => t.Namespace == "FluentNHibernate.Automapping.TestFixtures")
    .Override<ExampleClass>(t => t.DiscriminateSubClassesOnColumn("Discriminator"))
    .Override<ExampleInheritedClass>(m => m.DiscriminatorValue(1));