Allow FloatingActionButtons to be layed out in a ViewGroup inside a FloatingActionMenu

Children of the `FloatingActionMenu` that are `ViewGroup`s are aligned such that the `FloatingActionButton` contained within the child is center-aligned with the menu button.

All other non-`ViewGroup` children are center-aligned with the menu button as before - their behavior is unchanged.

Only one level of nesting is supported.

Closes #22.