Added OrganizationFields property to Organization

Custom fields on organizations can now be fetched and edited using the existing code. No additional test code has been added (since i don't know how to add a meaningfull test case). Please feel free to add it if you can.

Access to the custom fields can be done using this approach:
Reading:
            ZendeskApi api = new ZendeskApi(site, user, password);
            IndividualOrganizationResponse org = api.Organizations.GetOrganization(<someid>);
            org.Organization.Notes = "TEST";

            MessageBox.Show(org.Organization.OrganizationFields["plugins"]);

Writing:
            org.Organization.OrganizationFields["plugins"] = "TESTVALUE";

Where "plugins" is the name of the key for the custom organization field.