Changing the output of ElementAPI
Update:

Looks like changing the 'data' key is a bit more difficult I suspected.  We just released Element API 1.3.0 that exposes the internal $resourceKey necessary to change that value.

In your elementapi.php config file, something like this should work:

UPDATE: This is the final code that I used, and it works perfectly.

return [
    'defaults' =&gt; [
        'resourceKey' =&gt; 'manifest'
    ],

    'endpoints' =&gt; [
        'assets/data/assets.json' =&gt; [
            'paginate' =&gt; false,
            'elementType' =&gt; ElementType::Asset,

            'transformer' =&gt; function(AssetFileModel $asset) {
                return [
                    'id' =&gt; $asset-&gt;title,
                    'src' =&gt; $asset-&gt;url,
                ];
            },
       ],
    ]
];




The output you're seeing is defined by your craft/config/elementapi.php config file.

You can read about that configuration file and all of the available settings in the Element API documentation at: https://github.com/craftcms/element-api/tree/v1#setup