Allow to include non standard properties in ModulePrefs

If i create a gadget spec like this:

<Module>
<ModulePrefs title="Test" size="small" appURL="http://appurl.com">
</ModulePrefs>
<Content type="html" href="test.html">
</Content>
</Module>

I can not see the properties "size" and "appURL", after request the metadata in javascript, it's because JsonRpcHandler is looking for specific properties in the line 216.

So all non standard properties will be ignored, already has a TODO in this line:

// TODO: This should probably just copy all data from
// ModulePrefs.getAttributes(), but names have to be converted to
// camel case.