begin work upgrading filters and views

This is all in flux at Podio right now, so I may have to hold off.

Begin work on making this kind of thing possible:

$filter = $app->filter;
$filter->limit(50)->offset(51)->fields['app-reference']->none();
$filter->fields['number']->from(5)->to(9);
$filter->pseudofields['created_by']->me();
foreach ($filter as $item) {
    …
}
fix iterating over references

foreach ($item->fields['reference'] as $ref) {
    $ref->fields['blah'] = 'hi';
    $ref->save();
}

until this commit, the code above had a subtle logic bug, where $ref
was a Chiara\PodioItem\Values\App object, and since this extends
Chiara\PodioItem\Field, the save() method was actually called field
save instead of app save.

Now the save() method works as one expects.

In addition, the logic was not preserving any changes made to the
object, this fixes that issue as well.
add convenience classes for card and table view
begin skeleton for unimplemented tasks docs