#48 - Form inputs are scrolled to incorrect position on iOS7 and under

When pinny is open and there are form elements in it, the pinny will scroll to the top when any input is focused.

Status: **Opened for visibility**

Reviewers: @scalvert @kpeatt 
Issue: https://github.com/mobify/pinny/issues/48

## Changes
- We scroll to the active element using Velocity after the bounce fix is performed
- If the active element has the attribute positionParent in data, Velocity will scroll to this element instead

## Notes
- positionParent is introduced because if the active element's parent has the css declaration 'position: relative', then the active element will return the incorrect value for Velocity to scroll to.
So this should be done inside the `_build` method rather than here. 

```
this.$spacer = $('<div />').addClass(classes.spacer).height(300).attr('hidden', 'hidden').appendTo(this.$content);
```

I would also call the class `pinny_spacer`