Create a unified method for creating hover popups.

* Added a function(s) to `RESUtils` to act as a unified handler for creating hover popups.
* Transitioned `showParent` and `subredditInfo` to the new handler.
* The ones in `userTagger` are being left until a later date when the rest of the tagger can be cleaned up.

```JavaScript
RESUtils.hover.begin(anchorElement, {/*configuration*/}, function(element, context) {
    this.resolve('Header HTML/DOM Element/jQuery Object', 'body stuff');
}, {/*context object*/});
```