Fix form not updating when reassigning. Feature for ignoring fields.

First of all, really useful library. Thx!

1) Form update did not work without reloading the tableView. You can test it in the example by tapping the submit button and toggling the line of the first commit in the FXForms.m. I needed some dynamics and this quick fix did it for me.

2) A possibility for ignoring fields would be really great. Consider a form with n properties. Introducing one non-form property would mean listing all the n others in the fields method. That turned out to be a little bit cumbersome and error-prone, especially if the properties are still changing a lot. The ignoredFields only has effect when the fields method is not implemented. I think this should not lead to a conflict. Or is there a better way? What do you think?