feat($ionicNavBarDelegate): showBar returns whether navbar is shown
feat($ionicNavBarDelegate): showBackButton returns whether bar is shown

Closes #1076

If the user for example switches tabs,
$ionicNavBarDelegate.getPreviousTitle() will return the title from the
navbar within the previous tab.  In this case, the back button will not
be shown.  To not use the previous title in a case like this, you can
now do:

```js
var shouldShowTitle = $ionicNavBarDelegate.showBackButton();
if (shouldShowTitle) {
  $scope.previousTitle = $ionicNavBarDelegate.getPreviousTitle();
}
```