feat($controller): disable using global controller constructors

With the exception of simple demos, it is not helpful to use globals
for controller constructors. This adds a new method to `$controllerProvider`
to re-enable the old behavior, but disables this feature by default.

BREAKING CHANGE:
`$controller` will no longer look for controllers on `window`.
The old behavior of looking on `window` for controllers was originally intended
for use in examples, demos, and toy apps. We found that allowing global controller
functions encouraged poor practices, so we resolved to disable this behavior by
default.

To migrate, register your controllers with modules rather than exposing them
as globals:

Before:

```javascript
function MyController() {
  // ...
}
```

After:

```javascript
angular.module('myApp', []).controller('MyController', [function() {
  // ...
}]);
```

Although it's not recommended, you can re-enable the old behavior like this:

```javascript
angular.module('myModule').config(['$controllerProvider', function($controllerProvider) {
  // this option might be handy for migrating old apps, but please don't use it
  // in new ones!
  $controllerProvider.allowGlobals();
}]);
```