Include element padding and margin in height calculation

[BnayaZil]

Since the reveal-react element wrapping the target element it should also include the element padding and margin (top and bottom) in his height calculation.

I think this issue can be resolved by modifying the following row: https://github.com/rnosov/react-reveal/blob/master/src/RevealBase.js#L158

p.s- I have no problem to create a PR if it's OK with you.