Created a display_input_accessory_view property for StringRow.

Default implementation displays a "Done" button to close the keyboard or input views.

# Background
Thanks so much for making Formotion!  It's such a pleasure to work with.

I've been creating forms with a lot of different kinds of text fields (dates, pickers, numbers, etc).  I ran into a slight usability issue in that I can't seem to find a way to close the inputView, example below:
![image](https://f.cloud.github.com/assets/157972/359892/ca00370e-a174-11e2-8f14-320404043e3a.png)


So I added an option to display an input accessory view with a "Done" button to close the input view, e.g:
![image](https://f.cloud.github.com/assets/157972/359923/f2dc783a-a175-11e2-8f6a-74f8529fcb9e.png)

I thought others might find it helpful as well.


# Usage
Add a _display_input_accessory_view_ property set to _true_, e.g:
```json
{
  title: "Date of Birth",
  key: :dob,
  type: :date,
  display_input_accessory_view: true
}
```