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 was not sure where to place this function.  To me it seems to belong in the template logic, because it is specific to the JSON format.  Thoughts or suggestions are welcome. :)