Adding support for virtuals

Enables the use of the following:

var User = Bookshelf.DB.Model.extend({
  tableName: 'users',
  virtualsInJSON: true,
  virtuals: {
    fullName: function() {
        return this.get('firstName') + ' ' + this.get('lastName');
    }
  }
});

var user = User.forge({firstName: 'Joe', lastName: 'Shmoe'});
console.log(user.fullName === 'Joe Shmoe'); // => true

var json = user.toJSON(); // will contain key `fullName` with value 'Joe Shmoe'