Allow for aligning popovers to their parent

By default, popovers align to the window. Sometimes it is
desired to have the popover align to its parent.

Now popovers can be described like this:

```hbs
{{#lio-popover alignToParent=true}}
  Awwwww yissssssssssssssss
{{/lio-popover}}
```

This is also configured to be true automatically for `lio-tip` since the popover is always within the tip.
So *that's* what this class was for.