Support for using AJAX as data source for Reveal popups.

Reveal looks awesome, but it's missing AJAX support for a long time. Here is what I've did:

1. Added `data-reveal-ajax` attribute, which can be used in 2 ways, when placed on a DOM node with `data-reveal-id` present already:
 * when it's value is `true`, then `href` attribute of DOM node is used as url for an AJAX request (useful for A tags)
 * when it's value is not `true`, then this is an url for AJAX request (useful for non-A tags)
2. When used through `$('.reveal-div').foundation('reveal', 'open')` allows to pass 3 argument, that will act as `settings` argument for underlying `$.ajax(settings)` class (comes in handy when a POST data needs to be sent long with request). For example:

```js
$('.reveal-div').foundation('reveal', 'open', {
  url: 'http://some-url',
  data: {param1: 'value1', param2: 'value2'}
});
```
Since Foundation also uses Zepto in some cases, then I'm not sure if following code is compatible with Zepto:
* https://github.com/aik099/foundation/commit/2e910dd343c68357da717efdae7f32cb4cf37c81#L0R103

Also based on code examples in #2088 I've added following `section` reflow for sections, that came from AJAX request (not sure if it's correct):
* https://github.com/aik099/foundation/commit/2e910dd343c68357da717efdae7f32cb4cf37c81#L0R142