added an `addObject` function to enable an add button
delete button now works
removed the error message from Lens
Major progress for #136. Doesn't work for singular editable-groups

The current format for <editable-group>s looks like this:

    <editable-group object="person" lens="employee.office_culture.event_participations">
        <editable property="date" label="date" type="date"></editable>
        <editable property="name" label="name"></editable>
        <editable property="url"  label="url" type="url"></editable>
    </editable-group>

TODO:

- [ ] make it work for singular sub properties
- [ ] consider making <editable>s work without <editable-groups>
fixes #136 although the code could be cleaner.

Works for both singular and plural embedded objects.
for #136 removes ignored arguments to `addObject` in editable_group.html