Fix JSON output

closes #76

 Remove dependency on the xml2json library

Instead add a function to convert an eventInstance into a PHP array, which can then be exported with json_encode.

The resulting json for an event instance will be slightly different, mostly in the way it represents items in list.
For example:

This XML structure used to translate into a Webpages.Webpage.URL (not an array).
```
<Webpages>
    <Webpage>
        <URL></URL>
    </Webpage>
 <Webpages>
```

The issue was that if the list contained multiple `Webpage`s, it the resulting JSON would become an array.  This was not apparent to clients that consumed the JSON response and could easily result in errors.  For example: logic like this was required: https://github.com/unl/wdntemplates/blob/ef0dede2fe548454a3171f448799c91cfe2231ef/wdn/templates_4.0/scripts/events-band.js#L28

To fix this, all lists of instances are now an array.

I would avoid all instances of `htmlspecialchars`. JSON does not need this kind of escaping and a client _would_ be expected to do their own escaping based on their client needs.