sidebar_image not shown on pages

On pages the sidbar_image is not shown, when set in the backend, as noted in issue #2.

`header.php` checks only for sidebar_images with the `article_custom_field()` function. However for pages this returns nothing, pages seem to need the `page_custom_field()` for that. I added both to the `header.php`.

I'm not sure if this is the best way to solve this problem, since I'm new to Anchor. However it seemed to worked that way. :) 

Cheers,
Jona