Expose FXFormField.options array as readonly property.

It is handy to obtain entire `options` array as readonly property. 

Sure there is `optionCount` and `optionAtIndex:...` which allows you to iterate through objects, but by accessing  `options` directly, the code will be more concise and easy to remember.

e.g.,

instead of 

```
  for (int i = 0; i < optionCount; i++) {
      id option = [field optionAtIndex:i];
      // do something with option
  }
```
we can do 
```
  for id option in field.options {
      // do something with option
  }
```
It is a subtle difference, but feel much better if you use in quite a few places.