feat($compile/ngBind): don't add binding info if not enabled

The compiler and ngBind directives add binding information (`ng-binding`
CSS class and `$binding` data object) to elements when they are bound to
the scope. This is only to aid testing and debugging for tools such as
Protractor and Batarang. In production this is unneccesary and add a
performance penalty.

This change disables adding this binding information by default. You can
enable it by calling `$compilerProvider.enableBindingInfo()` in a module
`config()` block.

BREAKING CHANGE:

The `ng-binding` CSS class and the `$binding` data object are no longer
attached to DOM elements that have a binding to the scope, by default.
If you relied on this feature then you can re-enable it in one of your
application modules:

```
someModule.config(['$compileProvider', function($compileProvider) {
  $compileProvider.enableBindingInfo();
}]);
```
since compile fn is not doing anything, we should change this directive to return `link` directly