Allow custom height through FXFormControllerDelegate

Currently, it is not possible to specify different height for the same custom cell class. 
(The only way to customize height is through `[FXFormCell heightForField:width:]` **class** method.)

`tableView:heightForRowAtIndexPath:` method is a `UITableViewDelegate` method, so it should be overridable through `FXFormControllerDelegate` like some of `UITableViewDelegate` methods. (e.g., tableView:viewForHeaderInSection:, tableView:heightForHeaderInSection: etc.)