Allow timed delay which is cancelled by the mouse re-entering. Fixes #48.

This feature allows developers to provide a "grace period" where the mouse may leave the body for a short time without immediately triggering the modal - it will only appear if the mouse does not re-enter after `delay` ms.

This new feature also fixes issue #48.  When you mouse into the page from the header, Windows Chrome 35+ seems to trigger these events in quick succession:

```
mouseenter
mouseleave
mouseenter
```

That middle `mouseleave` would normally cause the modal to appear;  however, due to the new feature, the subsequent `mouseenter` will cancel that timer and thus the modal won't appear.  (This even works if `delay` is set to `0`)

This fixes issue #48 for me on `Windows 7 64-bit Chrome 36.0.1985.103 beta-m`.