executor: remove `Base` arg from model function

Simplify the contract for functions exported by `models/*.js` files
by removing the second argument `Base`. The base class can be accessed
using `ModelCtor.base`.

An updated example of a model js file:

```js
module.exports = function(Customer) {
  Customer.setup = function() {
    Customer.base.setup.apply(this, arguments);
    // etc.
  };
};
```

lib/executor.js

@@ -156,8 +156,7 @@ function defineModels(app, instructions) {
         var code = require(data.sourceFile);
         if (typeof code === 'function') {
           debug('Customizing model %s', name);
-          // NOTE model.super_ is set by Node's util.inherits
+          code(model);
-          code(model, model.super_);
         } else {
           debug('Skipping model file %s - `module.exports` is not a function',
             data.sourceFile);

test/executor.test.js

@@ -56,9 +56,9 @@ describe('executor', function() {
 
   it('defines and customizes models', function() {
     appdir.writeFileSync('models/Customer.js', 'module.exports = ' +
-      function(Customer, Base) {
+      function(Customer) {
         Customer.settings._customized = 'Customer';
-        Base.settings._customized = 'Base';
+        Customer.base.settings._customized = 'Base';
       }.toString());
 
     boot.execute(app, someInstructions({

test/fixtures/browser-app/models/customer.js

@@ -1,4 +1,4 @@
-module.exports = function(Customer, Base) {
+module.exports = function(Customer) {
   Customer.settings._customized = 'Customer';
-  Base.settings._customized = 'Base';
+  Customer.base.settings._customized = 'Base';
 };