Add EZFormInputAccessoryTypeDone (hides prev/next input accessory buttons)

I added additional `EZFormInputAccessoryType`s (`EZFormInputAccessoryTypeDone` and `EZFormInputAccessoryTypeDoneLeftAligned`) to give the option of only showing a Done button in a form's input accessory view. The standard input accessory type also has previous and next buttons, which aren't necessary in forms where only one field can become the first responder.

Here's the situation where these new types would be useful:

You have a form with multiple fields where `canBecomeFirstResponder` equals `NO` (and these fields are therefore skipped over by the previous/next buttons in the input accessory view). For example: `EZFormRadioField`, `EZFormBooleanField` and `EZFormContinuousField`.

You have only one field in the form where `canBecomeFirstResponder == YES`, and the input view for this field cannot dismiss itself. For example:

- `EZFormTextField` where keyboard has no Done/Return button (e.g. num pad)
- `EZFormRadioField` with a `UIPickerView` input view
- `EZFormDateField` with a `UIDatePicker` input view
- `UITextView` where Return adds a new line instead of dismissing the keyboard

In these cases it's useful to have a Done button to dismiss the input view, but the previous/next buttons on the input accessory view are unnecessary.

I'm not sure if this situation occurs all that often, but I needed these extra `EZFormInputAccessoryType`s in my current project (where I have a form with multiple radio fields and one text field). Any feedback would be great. Thanks!